fix: use parent project name for worktree observation writes (#1820)

* fix: use parent project name for worktree observation writes (#1819)

Observations and sessions from git worktrees were stored under
basename(cwd) instead of the parent repo name because write paths
called getProjectName() (not worktree-aware) instead of
getProjectContext() (worktree-aware). This is the same bug as
#1081, #1317, and #1500 — it regressed because the two functions
coexist and new code reached for the simpler one.

Fix: getProjectContext() now returns parentProjectName as primary
when in a worktree, and all four write-path call sites now use
getProjectContext().primary instead of getProjectName().

Includes regression test that creates a real worktree directory
structure and asserts primary === parentProjectName.

* fix: address review nitpicks — allProjects fallback, JSDoc, write-path test

- ContextBuilder: default projects to context.allProjects for legacy
  worktree-labeled record compatibility
- ProjectContext: clarify JSDoc that primary is canonical (parent repo
  in worktrees)
- Tests: add write-path regression test mirroring session-init/SessionRoutes
  pattern; refactor worktree fixture into beforeAll/afterAll

* refactor(project-name): rename local to cwdProjectName and dedupe allProjects

Addresses final CodeRabbit nitpick: disambiguates the local variable
from the returned `primary` field, and dedupes allProjects via Set
in case parent and cwd resolve to the same name.

---------

Co-authored-by: Ethan Hurst <ethan.hurst@outlook.com.au>

tests/utils/project-name.test.ts

@@ -5,7 +5,7 @@
  * Source: src/utils/project-name.ts
  */
 
-import { describe, it, expect } from 'bun:test';
+import { describe, it, expect, beforeAll, afterAll } from 'bun:test';
 import { homedir } from 'os';
 import { getProjectName, getProjectContext } from '../../src/utils/project-name.js';
 
@@ -96,4 +96,51 @@ describe('getProjectContext', () => {
     expect(ctx.primary).toBe('unknown-project');
     expect(ctx.parent).toBeNull();
   });
+
+  describe('worktree regression (#1081, #1500, #1819)', () => {
+    let tmp: string;
+    let mainRepo: string;
+    let worktreeCheckout: string;
+
+    beforeAll(async () => {
+      const { mkdtempSync, mkdirSync, writeFileSync } = await import('fs');
+      const { join } = await import('path');
+      const { tmpdir } = await import('os');
+
+      tmp = mkdtempSync(join(tmpdir(), 'cm-wt-'));
+      mainRepo = join(tmp, 'main-repo');
+      const worktreeGitDir = join(mainRepo, '.git', 'worktrees', 'my-worktree');
+      worktreeCheckout = join(tmp, 'my-worktree');
+
+      mkdirSync(worktreeGitDir, { recursive: true });
+      mkdirSync(worktreeCheckout, { recursive: true });
+      writeFileSync(
+        join(worktreeCheckout, '.git'),
+        `gitdir: ${worktreeGitDir}\n`
+      );
+    });
+
+    afterAll(async () => {
+      const { rmSync } = await import('fs');
+      rmSync(tmp, { recursive: true, force: true });
+    });
+
+    it('uses parent project name as primary when in a worktree', () => {
+      const ctx = getProjectContext(worktreeCheckout);
+      expect(ctx.isWorktree).toBe(true);
+      expect(ctx.primary).toBe('main-repo');
+      expect(ctx.parent).toBe('main-repo');
+      expect(ctx.allProjects).toEqual(['main-repo', 'my-worktree']);
+    });
+
+    it('write-path call sites resolve to parent project in worktrees', () => {
+      // Mirrors the pattern used by session-init.ts and SessionRoutes.ts:
+      //   const project = getProjectContext(cwd).primary;
+      // This must resolve to the parent repo, not the worktree name,
+      // so observations are stored under the correct project.
+      const project = getProjectContext(worktreeCheckout).primary;
+      expect(project).toBe('main-repo');
+      expect(project).not.toBe('my-worktree');
+    });
+  });
 });