fix(cursor-hook): keep npx fallback under the postToolUse host budget

hooks.json grants postToolUse a 10s budget. The npx fallback branch
added a flat +5s on top of the inner 7s spawnSync budget, reaching
12s — past the host deadline. When the local gitnexus package is
absent and npx cold-starts, Cursor kills the hook before the child
finishes, so the augmentation result is always lost on exactly the
machines the fallback exists for.

Cap the fallback at 10000 - 2000 = 8000ms (host budget minus
headroom for node startup and the final stdout write) via
Math.min, and add a source-level regression test asserting the cap
wiring so the fallback can never exceed the host budget again.

gitnexus-cursor-integration/hooks/gitnexus-hook.cjs

@@ -339,6 +339,16 @@ function resolveCliPath() {
   }
 }
 
+// The Cursor host enforces hooks.json's postToolUse `timeout` (10s) against
+// the whole hook process. The npx fallback used to add a flat +5s on top of
+// the inner budget (7000 → 12000ms), past the host deadline, so Cursor killed
+// the hook before the child finished and the augmentation was always lost on
+// cold-start machines — the exact scenario the fallback exists for. Cap the
+// fallback under the host budget, leaving headroom for node startup and the
+// final stdout write.
+const CURSOR_HOST_BUDGET_MS = 10000;
+const CURSOR_NPX_HEADROOM_MS = 2000;
+
 function runGitNexusCli(cliPath, args, cwd, timeout) {
   const isWin = process.platform === 'win32';
   if (cliPath) {
@@ -350,9 +360,13 @@ function runGitNexusCli(cliPath, args, cwd, timeout) {
       windowsHide: true,
     });
   }
+  const npxTimeout = Math.min(
+    timeout + 5000,
+    CURSOR_HOST_BUDGET_MS - CURSOR_NPX_HEADROOM_MS,
+  );
   return spawnSync(isWin ? 'npx.cmd' : 'npx', ['-y', 'gitnexus', ...args], {
     encoding: 'utf-8',
-    timeout: timeout + 5000,
+    timeout: npxTimeout,
     cwd,
     stdio: ['pipe', 'pipe', 'pipe'],
     windowsHide: true,

gitnexus/test/unit/cursor-hook.test.ts

@@ -441,6 +441,25 @@ describe('Cursor hook debug logging', () => {
   });
 });
 
+// ─── Source code regression: npx fallback stays under the host budget ─────
+
+describe('Cursor hook npx fallback host budget', () => {
+  const source = fs.readFileSync(CURSOR_HOOK, 'utf-8');
+
+  it('caps the npx fallback timeout below the hooks.json postToolUse budget', () => {
+    // hooks.json grants postToolUse 10s. The fallback branch adds +5s on top
+    // of the inner 7s budget, which used to reach 12s — past the host
+    // deadline, so Cursor killed the hook and cold-start users never saw
+    // augmentation. Assert the cap wiring exists and the effective fallback
+    // budget (7000 + 5000, clamped by 10000 - 2000) can never exceed the
+    // host budget again.
+    expect(source).toContain('CURSOR_HOST_BUDGET_MS = 10000');
+    expect(source).toContain('CURSOR_NPX_HEADROOM_MS = 2000');
+    expect(source).toContain('Math.min(');
+    expect(source).not.toMatch(/timeout:\s*timeout \+ 5000/);
+  });
+});
+
 // ─── Source code regression: concurrency guard (#1486) ─────────────
 
 describe('Cursor hook concurrency guard', () => {