Improve git worktree support: hook installer + storage path collision

[fazleelahhee]

Context

CCE works per-directory, so each git worktree gets its own index under ~/.cce/projects/<basename>/. That covers the common case, but there are two known rough edges that make worktrees feel second-class.

1. Auto-reindex git hooks don't install in worktrees

install_hooks in src/context_engine/indexer/git_hooks.py:40-50 writes to <project_dir>/.git/hooks/:

hooks_dir = Path(project_dir) / ".git" / "hooks"
if not hooks_dir.exists():
    return []

In a git worktree, <project_dir>/.git is a file pointing at <main-repo>/.git/worktrees/<name>/, not a directory. The check fails silently and no hook is installed. Manual cce index still works, but post-commit/post-checkout/post-merge auto-reindex never fires from inside a worktree.

Fix: resolve the hooks dir via git itself instead of assuming layout:

result = subprocess.run(
    ["git", "rev-parse", "--git-path", "hooks"],
    cwd=project_dir, capture_output=True, text=True, check=True,
)
hooks_dir = Path(result.stdout.strip())
if not hooks_dir.is_absolute():
    hooks_dir = Path(project_dir) / hooks_dir

git rev-parse --git-path hooks returns the right directory in both regular checkouts and worktrees. Worktrees share the main repo's hooks by default, which is the right behavior — installing once from any worktree fires the hook for all of them.

2. Storage key is directory basename, not absolute path

Index storage resolves to Path(config.storage_path) / project_dir.name (see e.g. tests/indexer/test_pipeline_durability.py:38, cli.py callers). Two worktrees with the same basename — for example ~/work/myrepo and ~/scratch/myrepo — would clobber each other's index, embedding cache, and memory db.

The collision is unusual in practice because worktrees are typically given suffixed names (myrepo-feat-x, myrepo-bugfix-y), but the failure mode is silent and corrupts state when it does happen.

Fix: the codebase already solves this for editor configs in src/context_engine/editors.py:145-168 via a <basename>-<6hex> slug derived from Path.resolve(). Apply the same scheme to the storage key, with a one-time migration that moves an existing <basename>/ directory to <basename>-<hash>/ if and only if the resolved path matches.

Test plan

• Hook installs successfully when run from a worktree, using the right shared hooks dir
• Re-running cce install from any worktree is idempotent (no duplicate marker)
• Indexing two worktrees of the same repo with the same basename keeps separate stores
• Existing single-checkout users keep their current storage path (migration is no-op when basename is unique)

[fazleelahhee]

Splitting this into two for review and risk:

• Item 1 (git hooks worktree-aware) — fixed in fix(indexer): resolve git hooks dir via git, not hardcoded .git/hooks #53. Surgical change, ~10 lines of production code, clear behavior. Real worktree users were silently getting no auto-reindex.
• Item 2 (storage path slug to avoid basename collision) — moved to Storage path uses directory basename, not absolute path (worktree collision risk) #52. Touches 30+ call sites and needs a one-time on-disk migration for existing users, so it deserves its own focused PR.

Closing this one in favor of those two.