fix(projects): resolve git worktree .git files in detect_project

[GetsEclectic]

Problem (data loss)

Any trc create / close / update run from inside a git worktree
silently truncates the parent checkout's .trace/issues.jsonl to a per-worktree
(usually empty) subset — dropping every issue the main checkout had.

Reproduction:

cd repo                       # main checkout: N issues
git worktree add wt && cd wt
trc create "anything"         # any write-path command
wc -l ../.trace/issues.jsonl  # → 1   (N-1 issues silently lost)

This bites hard with parallel agents, each working in its own worktree.

Root cause

In a worktree, .git is a file containing gitdir: <path>, not a
directory. detect_project() walked up to that .git and Path.exists()
returned True, so it treated the worktree dir as a repo root:

• project_path became the worktree dir.
• _extract_*_from_git_remote(git_dir) opened git_dir / "config", which
  doesn't exist (git_dir is a file), so both returned None and the fallbacks
  assigned the worktree path/name as a brand-new project identity.

Every worktree thus registered as a new project, and the next write called
export_to_jsonl, which does path.open("w") and writes only the
(new, empty) worktree-project's issues — clobbering the real file.

Fix

Add _resolve_git_dir() which resolves a .git pointer file the way git
itself does — follow gitdir: → read commondir → locate the canonical
.git → return the main checkout root. detect_project() now uses it, so a
worktree resolves to the same id / name / path as the main checkout.
config is then read from the canonical .git, restoring correct remote-based
identity. Concurrent worktrees agree on one project identity, matching the
JSONL-line-merge model the project already relies on.

Tests

tests/test_worktree_detect_project.py:

1. _resolve_git_dir resolves a hand-built worktree pointer to canonical
   .git + main root (no git binary needed).
2. detect_project() from inside a real git worktree add returns identity
   identical to the main checkout (same id, name, and path).

Follow-up (separate PR)

Even with identity fixed, .trace/issues.jsonl still physically lives under
whatever .trace/ is in the current worktree. Teaching sync_project /
export_to_jsonl to always write to the project root's .trace/ would
fully de-race concurrent worktrees. Gated on this PR.