fix(group)!: remove the matching cascade that was advertised but never built

`gitnexus group create` wrote `matching.bm25_threshold` and
`matching.embedding_threshold` into every generated group.yaml, and no matcher
ever read either one. That was not the whole of it — an entire feature surface
described a BM25/embedding cascade that does not exist:

- `matching.bm25_threshold` / `matching.embedding_threshold` — parsed, persisted,
  unread
- `detect.embedding_fallback` — defaulted and templated, unread
- `MatchType` declared `'bm25' | 'embedding'`; both variants unreachable
- `SyncOptions.skipEmbeddings` — declared in sync.ts and never read
- `gitnexus group sync --skip-embeddings` — accepted, threaded through
  GroupService, ignored
- CLI help in en and zh-CN promised "Exact + BM25 only (no embedding fallback)"
- the MCP `group_sync` schema exposed `skipEmbeddings`, described as
  "Exact + BM25 only (Demo PR: same as default exact path)"

`sync.ts` imports exactly `buildProviderIndex`, `runExactMatch` and
`runWildcardMatch`, and the printed cascade has one stage. An operator whose
links do not match reaches for those thresholds first, and turning either knob
changes nothing — config that silently does nothing is how people conclude a
feature is broken.

Evidence that the cascade should be deleted rather than implemented, from a real
backend/frontend pair: of 165 consumer contracts, 149 link exactly and 16 do not.
Nine of the sixteen are third-party APIs (Google OAuth, Apple public keys,
PostHog, image annotation) with no in-group provider by construction — similarity
matching cannot recover them, it can only invent false links. Two are verb
mismatches: the frontend calls `POST /links` and `GET /links/check-exists` while
the backend declares `GET /links` and eleven other `/links/*` routes but neither
of those, so a fuzzy path match would link a POST consumer to a GET provider. The
rest are path-extraction artifacts. Roughly none of the sixteen would be
correctly recovered, and several would be actively mis-linked.

BREAKING CHANGE: `gitnexus group sync --skip-embeddings` and the MCP `group_sync`
`skipEmbeddings` parameter are removed. Both were accepted and ignored, so no
behavior changes — but a script passing the flag now fails with `unknown option`
instead of being silently misled. Existing group.yaml files keep loading: the
removed keys are simply no longer part of the schema, and a regression test pins
that a legacy config carrying all three still parses.

Closes #3006

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

gitnexus/src/cli/group.ts

@@ -149,7 +149,6 @@ export function registerGroupCommands(program: Command): void {
   group
     .command('sync <name>')
     .description('Sync Contract Registry — extract contracts and build cross-links')
-    .option('--skip-embeddings', 'Exact + BM25 only (no embedding fallback)')
     .option('--exact-only', 'Exact match only')
     .option('--allow-stale', 'Skip stale index warnings')
     .option('--verbose', 'Show each cross-link detail')
@@ -168,7 +167,6 @@ export function registerGroupCommands(program: Command): void {
         groupDir,
         allowStale: Boolean(opts.allowStale),
         verbose: Boolean(opts.verbose),
-        skipEmbeddings: Boolean(opts.skipEmbeddings),
         exactOnly: Boolean(opts.exactOnly),
       });
 

gitnexus/src/cli/help-i18n.ts

@@ -155,7 +155,6 @@ const OPTION_DESCRIPTION_KEYS = {
   'embeddings install|--cuda': 'help.option.embeddings.install.cuda',
   'embeddings install|--force': 'help.option.embeddings.install.force',
   'group create|--force': 'help.option.group.create.force',
-  'group sync|--skip-embeddings': 'help.option.group.sync.skipEmbeddings',
   'group sync|--exact-only': 'help.option.group.sync.exactOnly',
   'group sync|--allow-stale': 'help.option.group.sync.allowStale',
   'group sync|--verbose': 'help.option.group.sync.verbose',

gitnexus/src/cli/i18n/en.ts

@@ -293,7 +293,6 @@ export const en = {
   'help.option.embeddings.install.force':
     'Install into the runtime prefix even when the stack already resolves',
   'help.option.group.create.force': 'Overwrite existing group',
-  'help.option.group.sync.skipEmbeddings': 'Exact + BM25 only (no embedding fallback)',
   'help.option.group.sync.exactOnly': 'Exact match only',
   'help.option.group.sync.allowStale': 'Skip stale index warnings',
   'help.option.group.sync.verbose': 'Show each cross-link detail',

gitnexus/src/cli/i18n/zh-CN.ts

@@ -273,7 +273,6 @@ export const zhCN = {
     '同时下载 CUDA GPU 二进制文件（运行 onnxruntime-node 的 NuGet postinstall；代理后请设置 GLOBAL_AGENT_HTTPS_PROXY）',
   'help.option.embeddings.install.force': '即使嵌入组件已可解析，也强制安装到运行时目录',
   'help.option.group.create.force': '覆盖现有仓库组',
-  'help.option.group.sync.skipEmbeddings': '仅使用 exact + BM25（不使用嵌入回退）',
   'help.option.group.sync.exactOnly': '仅精确匹配',
   'help.option.group.sync.allowStale': '跳过过期索引警告',
   'help.option.group.sync.verbose': '显示每条跨仓库链接详情',

gitnexus/src/core/group/config-parser.ts

@@ -30,14 +30,11 @@ const DEFAULT_DETECT = {
   thrift: true,
   topics: true,
   shared_libs: true,
-  embedding_fallback: true,
   includes: false,
   workspace_deps: false,
 };
 
 const DEFAULT_MATCHING = {
-  bm25_threshold: 0.7,
-  embedding_threshold: 0.65,
   max_candidates_per_step: 3,
   exclude_links_paths: [] as string[],
   exclude_links_param_only_paths: false,

gitnexus/src/core/group/service.ts

@@ -350,7 +350,6 @@ export class GroupService {
     const result = await syncGroup(config, {
       groupDir,
       exactOnly: Boolean(params.exactOnly),
-      skipEmbeddings: Boolean(params.skipEmbeddings),
       allowStale: Boolean(params.allowStale),
       verbose: Boolean(params.verbose),
     });

gitnexus/src/core/group/storage.ts

@@ -94,11 +94,8 @@ detect:
   grpc: true
   topics: true
   shared_libs: true
-  embedding_fallback: true
 
 matching:
-  bm25_threshold: 0.7
-  embedding_threshold: 0.65
   max_candidates_per_step: 3
   # exclude_links_paths: [/ping, /health, /healthcheck]
   # exclude_links_param_only_paths: false

gitnexus/src/core/group/types.ts

@@ -1,5 +1,5 @@
 export type ContractType = 'http' | 'grpc' | 'thrift' | 'topic' | 'lib' | 'custom' | 'include';
-export type MatchType = 'exact' | 'manifest' | 'wildcard' | 'bm25' | 'embedding';
+export type MatchType = 'exact' | 'manifest' | 'wildcard';
 export type ContractRole = 'provider' | 'consumer';
 
 export interface GroupConfig {
@@ -27,14 +27,11 @@ export interface DetectConfig {
   thrift: boolean;
   topics: boolean;
   shared_libs: boolean;
-  embedding_fallback: boolean;
   includes: boolean;
   workspace_deps: boolean;
 }
 
 export interface MatchingConfig {
-  bm25_threshold: number;
-  embedding_threshold: number;
   max_candidates_per_step: number;
   /**
    * HTTP paths to exclude from cross-link matching. Contracts at these paths

gitnexus/src/mcp/tools.ts

@@ -859,10 +859,6 @@ WHEN TO USE: After changing group.yaml or re-indexing member repos.`,
       type: 'object',
       properties: {
         name: { type: 'string', description: 'Group name' },
-        skipEmbeddings: {
-          type: 'boolean',
-          description: 'Exact + BM25 only (Demo PR: same as default exact path)',
-        },
         exactOnly: { type: 'boolean', description: 'Exact match only in cascade' },
       },
       required: ['name'],

gitnexus/test/unit/group/config-parser.test.ts

@@ -59,11 +59,36 @@ repos:
     expect(config.links).toEqual([]);
     expect(config.packages).toEqual({});
     expect(config.detect.http).toBe(true);
-    expect(config.matching.bm25_threshold).toBe(0.7);
+    expect(config.matching.max_candidates_per_step).toBe(3);
     expect(config.matching.exclude_links_paths).toEqual([]);
     expect(config.matching.exclude_links_param_only_paths).toBe(false);
   });
 
+  it('still parses a legacy config carrying the removed matching knobs', () => {
+    // `bm25_threshold`, `embedding_threshold` and `detect.embedding_fallback`
+    // were written into every generated group.yaml but read by no matcher, so
+    // they are gone from the schema and the template. Every group.yaml already
+    // on disk still has them, and must keep loading without complaint.
+    const legacy = `
+version: 1
+name: test
+repos:
+  app: my-app
+detect:
+  http: true
+  embedding_fallback: true
+matching:
+  bm25_threshold: 0.7
+  embedding_threshold: 0.65
+  max_candidates_per_step: 3
+`;
+    const config = parseGroupConfig(legacy);
+    expect(config.name).toBe('test');
+    expect(config.repos).toEqual({ app: 'my-app' });
+    expect(config.detect.http).toBe(true);
+    expect(config.matching.max_candidates_per_step).toBe(3);
+  });
+
   it('defaults thrift detection to true', () => {
     const minimal = `
 version: 1