fix: resolve worktrees whose recorded gitdir path exists on the host

[rosstaco]

Problem

idc doctor reported a valid git worktree as an unsupported external worktree on a WSL remote:

⚠ worktree: /home/user/repos/repo/.worktrees/wt-1 looks like a worktree or submodule but cannot be resolved (external worktree or submodule)

Root cause

A worktree created inside a devcontainer records a container-absolute path in its .git file:

gitdir: /workspaces/repo/.worktrees/wt-1

resolve_worktree() derived the main repo from that recorded path and only walked the local filesystem when the derived path didn't exist. On WSL, /workspaces/agent-spendster happened to exist as an unrelated directory, so:

• main_repo.is_dir() returned True → the local-filesystem fallback was skipped
• target.relative_to(main_repo) then raised ValueError
• → the worktree was rejected as "external"

Fix

Locate the owning repository on the local filesystem first (walk up from the worktree), and trust the recorded gitdir path only as a fallback when no local ancestor repository is found. The recorded path can't be trusted — it may be foreign (a container path) or even coincidentally present as an unrelated directory.

Tests

• New regression test test_resolves_worktree_when_foreign_gitdir_path_exists_on_host reproduces the exact WSL failure — fails on old code, passes on new.
• Full suite: 348 passed; ruff clean.