perf(ci): cache node_modules, agent .venv, and Jest transforms in build workflow

[scottschreckengaust]

Summary

• Adds actions/cache@v4 for node_modules (keyed on yarn.lock), agent/.venv (keyed on uv.lock), and Jest transform cache (keyed on yarn.lock + commit SHA with cross-branch restore-keys)
• Sets Jest cacheDirectory to <rootDir>/.jest-cache — moves cache off tmpfs (fixes /tmp filling), enables persistence locally and in CI
• Adds .jest-cache to .gitignore
• Includes timing instrumentation (SECONDS + ::notice) to measure cache-hit vs cold performance

How to measure the improvement

The workflow logs cache status and timing in GHA annotations:

::notice::Install completed in 5s (node_modules=true, venv=true, jest=true)   ← all cached
::notice::Install completed in 75s (node_modules=miss, venv=miss, jest=miss)  ← cold
::notice::Build completed in 580s                                             ← with jest cache
::notice::Build completed in 725s                                             ← without jest cache

To compare:

1. First run: all caches MISS (cold baseline)
2. Second run (re-run or push): caches HIT — install drops to ~5s, build drops ~30-50s
3. Compare via ::notice annotations in workflow summary

To drop the instrumentation after validation: Remove the SECONDS=0 / echo "::notice::" lines. The caching is the permanent change.

Cache strategy

Cache	Key	Restore-keys	Invalidates when
node_modules	node-modules-{os}-{hash(yarn.lock)}	None	Lockfile changes
agent/.venv	agent-venv-{os}-{hash(agent/uv.lock)}	None	Python deps change
Jest transforms	jest-{os}-{hash(yarn.lock)}-{sha}	jest-{os}-{hash(yarn.lock)}-	Always unique per commit; falls back to same-lockfile prior run

Jest cache uses restore-keys because partial cache (most files unchanged) is still beneficial. The others don't, because stale node_modules/.venv with wrong versions would break the build.

Expected impact

Metric	Before (cold)	After (cache hit)
Install step	~75s	~5-10s
Jest test step	~90s	~40-50s
Total wall time saved	—	~110-120s

Also fixes: Jest transform cache no longer fills /tmp tmpfs (was causing ENOSPC failures locally).

Test plan

• First PR run: verify all caches MISS, timings match baseline
• Second run: verify caches HIT, install ~5s, build faster
• Verify mise run build still passes
• Verify mutation detection still works
• Verify .jest-cache/ does not appear in git status

Refs #201

🤖 Generated with Claude Code

Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com