fix: resolve git origin in linked worktrees (Tia storage key + baseline fetch)

[dammyammy]

Problem

In a linked git worktree, .git is a file (a gitdir: pointer), not a directory. Two Tia code paths read $projectRoot/.git/config directly and therefore silently fail in every worktree:

1. Storage::rawOriginUrl() — the storage key falls back to a realpath hash instead of the origin identity. The docs state worktrees of the same repository share one cache; today they don't.
2. BaselineSync::detectGitHubRepo() — returns null, so --tia --baselined silently skips the CI baseline fetch and falls back to a full local re-record, exactly in the throwaway checkouts (agent/CI worktrees) where a fetched baseline helps most.

Measured on v5.0.2:

# main checkout
$ vendor/bin/pest --baseline
/Users/x/.pest/tia/repo-840a07cab1e2d72d        # origin-derived hash

# linked worktree of the same repo (before this fix)
$ vendor/bin/pest --baseline
/Users/x/.pest/tia/wt-feee7be378801f90          # realpath hash — origin ignored

# linked worktree (after this fix)
$ vendor/bin/pest --baseline
/Users/x/.pest/tia/wt-840a07cab1e2d72d          # origin-derived hash again

Fix

Keep the fast .git/config file read for regular checkouts; when .git is not a directory, fall back to git config --get remote.origin.url executed inside the project root (5s timeout, failures degrade to the existing null behaviour).

Notes for review

• Storage::projectKey() still prefixes slug(basename($projectRoot)), so two worktrees end up with sibling storage dirs (wt-a-<hash>, wt-b-<hash>) rather than literally one shared dir. If full sharing is the intent of the docs, the slug would also need to derive from the origin identity — happy to extend the PR that way if preferred; keeping keys per-worktree does avoid concurrent runs in different worktrees racing on one graph.json.
• With this fix, --baselined works from worktrees, which covers the main practical need (fresh worktrees replaying from the CI baseline in seconds).

Draft — description/scope still being refined.