perf(tests): per-worker template repo cuts Windows CI fixture cost

[pbean]

Why

Follow-up to #42. Windows test jobs only scaled ~1.5× under xdist (265s vs 412s serial): the project fixture git-inits a fresh repo per test — 5 subprocess spawns × 412 tests — and on Windows that's ~3s of setup per test (slow process spawn + fsync, 4 workers contending on disk). The --durations table on the #42 run showed exactly this: ~3s setup entries for test_verify_worktree/test_cli/test_engine.

What

tests/conftest.py only:

• New session-scoped _project_template fixture builds the identical sandbox once per xdist worker (same files, same 5 git commands, identity kept in local config so copies inherit it).
• project now shutil.copytrees the template — zero git spawns per test (~25 small files, tens of ms on Windows).
• core.fsync=none in the template's local config, inherited by copies and their worktrees, cheapening the ~100 remaining inline add/commit calls.

Safety, verified against the code

• No phantom-dirty from the copied index: all cleanliness checks in verify.py are porcelain (status --porcelain, git diff --quiet, OID compare, throwaway GIT_INDEX_FILE) which re-hash stat-dirty entries; diff-index/diff-files plumbing appears nowhere. Also proven empirically: a copytree'd repo reports clean porcelain status.
• Fresh non-worktree repos contain no absolute paths → fully relocatable; .git/worktrees/* is created per-copy, so the real git worktree add tests are unaffected.
• All rev_parse_head assertions are same-repo; identical initial SHAs across copies are harmless.
• The identity-unset test (test_verify.py) --unsets local config, which the copy carries exactly as before.

Verified locally

Full suite green serial (1321 passed, 108s — down from ~125s) and 3× under -n 4/-n auto (28–33s). Targeted canaries green: identity-unset + finalize-commit + commit-paths, all worktree suites (103 passed), ledger-commit suites (77 passed). --durations=15 now shows no setup entries at all. trunk check clean.

Expected on this PR's Windows jobs: test phase ~265s → ~100–150s. The one ~3s template build per worker shows up on whichever test runs first — by design.

🤖 Generated with Claude Code

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• tests/conftest.py

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🤖 Augment PR Summary

Summary: Refactors the project test fixture to build a sandbox git repo template once per pytest session/xdist worker and copy it per test, eliminating expensive Windows setup.

Changes: Introduces a session-scoped _project_template and makes project use shutil.copytree, inheriting local git config (identity + core.fsync=none) into each disposable test repo.

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