feat(v0.6): per-project detail view — branch/worktrees + active sessions

Clicking a project's name opens an in-app detail view (back button returns to
the grid) with:
- header: name, path, branch, dirty
- Branch / Worktrees: each `git worktree list` entry (branch or detached@head,
  main/current/locked/prunable tags, path) — a single-worktree repo shows its
  branch; a multi-worktree repo shows them all
- Active sessions (N): detailed rows (status, devices watching, started-ago)
  with Open + Kill
- the Claude/Codex/VS Code launcher row

Backend: src/http/worktrees.ts (parseWorktrees pure + listWorktrees via
execFile, no shell, best-effort); buildProjectDetail(cfg, path, liveSessions)
in projects.ts (validates absolute existing dir, reuses branch/dirty/session
helpers, uncached — detail is on-demand and wants fresh state); read-only
GET /projects/detail?path= (no Origin guard, like /projects; 400 missing /
404 invalid). Types: WorktreeInfo, ProjectDetail.

Frontend: detail view + navigation in projects.ts (grid<->detail), project
name becomes a link, Kill via DELETE /live-sessions/:id.

Tests +21 (452 total): worktree parser, buildProjectDetail (validation,
non-git, session merge, real git-init worktree), renderProjectDetail.
worktrees.ts 100% / projects.ts 93.8% cov. Verified in-browser: opened a
Claude session, then the detail showed its branch+worktree and the live
session with Open/Kill.

test/projects.test.ts

@@ -14,7 +14,9 @@ vi.mock('../src/http/history.js', () => ({
   listSessions: (...a: unknown[]) => mockListSessions(...a),
 }))
 
-const { parseGitHead, buildProjects, _clearProjectCache } = await import('../src/http/projects.js')
+const { parseGitHead, buildProjects, buildProjectDetail, _clearProjectCache } = await import(
+  '../src/http/projects.js'
+)
 
 const execFileP = promisify(execFile)
 
@@ -347,3 +349,73 @@ describe('buildProjects — dirty check', () => {
     expect(out.find((p) => p.name === 'repoA')!.dirty).toBeUndefined()
   })
 })
+
+// ── buildProjectDetail ───────────────────────────────────────────────────────────
+
+describe('buildProjectDetail', () => {
+  it('returns null for a relative path', async () => {
+    expect(await buildProjectDetail(makeCfg({}), 'relative/dir', [])).toBeNull()
+  })
+
+  it('returns null for a non-existent path', async () => {
+    expect(await buildProjectDetail(makeCfg({}), path.join(tmp, 'nope'), [])).toBeNull()
+  })
+
+  it('returns null when the path is a file', async () => {
+    const f = path.join(tmp, 'file.txt')
+    await fs.writeFile(f, 'x')
+    expect(await buildProjectDetail(makeCfg({}), f, [])).toBeNull()
+  })
+
+  it('reports a non-git directory with no worktrees', async () => {
+    const dir = path.join(tmp, 'plain')
+    await fs.mkdir(dir)
+    const d = await buildProjectDetail(makeCfg({}), dir, [])
+    expect(d).not.toBeNull()
+    expect(d!.isGit).toBe(false)
+    expect(d!.worktrees).toEqual([])
+    expect(d!.name).toBe('plain')
+  })
+
+  it('reads the branch for a git repo (worktrees best-effort)', async () => {
+    const dir = path.join(tmp, 'repo')
+    await makeRepo(dir, 'develop')
+    const d = await buildProjectDetail(makeCfg({}), dir, [])
+    expect(d!.isGit).toBe(true)
+    expect(d!.branch).toBe('develop')
+    expect(Array.isArray(d!.worktrees)).toBe(true) // real `git` fails on the fake .git → []
+  })
+
+  it('merges running sessions whose cwd is the project or under it', async () => {
+    const dir = path.join(tmp, 'repo')
+    await fs.mkdir(dir)
+    const live = [
+      makeLive({ id: 'a', cwd: dir, status: 'working' }),
+      makeLive({ id: 'b', cwd: path.join(dir, 'sub') }),
+      makeLive({ id: 'c', cwd: path.join(tmp, 'other') }),
+    ]
+    const d = await buildProjectDetail(makeCfg({}), dir, live)
+    expect(d!.sessions.map((s) => s.id).sort()).toEqual(['a', 'b'])
+  })
+
+  it('lists worktrees for a real git repo (gated on git)', async () => {
+    if (!(await gitAvailable())) return
+    await fs.mkdir(path.join(tmp, 'realrepo'))
+    // git canonicalises paths (/var → /private/var on macOS), so pass the
+    // realpath — real project paths from discovery are already canonical.
+    const dir = await fs.realpath(path.join(tmp, 'realrepo'))
+    await execFileP('git', ['init', '-q', '-b', 'main'], { cwd: dir })
+    await execFileP('git', ['config', 'user.email', 't@t'], { cwd: dir })
+    await execFileP('git', ['config', 'user.name', 't'], { cwd: dir })
+    await fs.writeFile(path.join(dir, 'f.txt'), 'hi')
+    await execFileP('git', ['add', '.'], { cwd: dir })
+    await execFileP('git', ['commit', '-q', '-m', 'init'], { cwd: dir })
+
+    const d = await buildProjectDetail(makeCfg({ projectDirtyCheck: true }), dir, [])
+    expect(d!.isGit).toBe(true)
+    expect(d!.branch).toBe('main')
+    expect(d!.worktrees.length).toBeGreaterThanOrEqual(1)
+    expect(d!.worktrees[0]).toMatchObject({ isMain: true, isCurrent: true, branch: 'main' })
+    expect(d!.dirty).toBe(false)
+  })
+})