test(hooks): make the pre-push tsx stub path survive .git-less cloud image checkouts

[danshapiro]

Summary

The Cloud Run vitest lane has been silently hanging since the pre-push hook
routing tests landed (ba83dd4, Sep 19): every cloud test:unit run wedges
one shard forever and fails at the 30-minute job timeout with zero test
failures. This breaks every agent's green-base gate and full-suite run on
the cloud backend (two scripts/base-gate.sh test attempts at 8dea394 both
went red this way: 539/540 test files green, one file never completing, in
both runs).

Root cause

test/unit/scripts/rust-test-targets.test.ts derives the real tsx binary
for its hook fixture from the git common dir:

1. The Cloud Run image copies the source tree without .git
   (.dockerignore excludes it), so the module-level
   git rev-parse --git-common-dir probe fails with empty stdout.
2. The old code fell back to path.join('', ...) — the relative
   node_modules/.bin/tsx.
3. The fixture stub is #!/bin/sh exec "<path>" "$@"; invoked from the
   fixture repo, a relative path re-execs the stub itself in an infinite
   silent loop (no output, no failure).
4. spawnSync had no timeout, so the vitest worker blocks until the job
   timeout reaps the shard.

Locally this never reproduces (the checkout has .git).

Verified in the exact production image
(freshell-e2e:8dea394d996b, --network=none, worktree file overlaid):

• git rev-parse from /app/test/unit/scripts: exit 128, empty stdout.
• The relative-path stub loops (killed only by timeout, exit 124).
• The original test file wedges vitest so hard it never finishes —
  it survives its own 90-second SIGTERM deadline (the container never
  exits).
• The fixed file: 29/29 in 3.5s in the same image.

Fix (test-only)

• owningTsxPath(): fall back to the checkout root containing the test
  file when git metadata is absent, and force an absolute result so the
  stub can never self-exec-loop.
• spawnSync timeouts (15s/30s/120s) so a wedged child fails the test in
  bounded time instead of eating the 30-minute job.

No production code changes.

Verification

• New unit tests pin both the git-present behavior and the no-git fallback
  (red → green).
• Full default-config local unit suite green (489 files), including the
  fixed file's 29 tests.
• Cloud Run vitest lane on this branch: green (4 shards, image built at
  05a89bb).

Test plan

• npm run test:vitest -- run test/unit/scripts/rust-test-targets.test.ts --config config/vitest/vitest.config.ts — 29/29.
• Cloud lane on the branch: FRESHELL_VITEST_BACKEND=cloud npm run test:unit — exit 0.