Add arc-linear crate for Linear issue tracker API integration

GraphQL client with 6 public functions: execute_graphql, fetch_viewer_id,
create_comment, update_issue_state, fetch_candidate_issues, and
fetch_issues_by_ids. Includes Issue/BlockerRef domain types with manual
normalization from GraphQL JSON responses.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -314,6 +314,18 @@ dependencies = [
  "tracing",
 ]
 
+[[package]]
+name = "arc-linear"
+version = "0.2.0"
+dependencies = [
+ "mockito",
+ "reqwest 0.12.28",
+ "serde",
+ "serde_json",
+ "tokio",
+ "tracing",
+]
+
 [[package]]
 name = "arc-llm"
 version = "0.2.0"

lib/crates/arc-linear/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "arc-linear"
+edition.workspace = true
+version.workspace = true
+license.workspace = true
+description = "Linear issue tracker API helpers for Arc"
+
+[lib]
+doctest = false
+
+[dependencies]
+serde.workspace = true
+serde_json.workspace = true
+reqwest.workspace = true
+tracing.workspace = true
+
+[dev-dependencies]
+mockito = "1"
+tokio = { workspace = true, features = ["test-util", "macros"] }