ci: parallelize quality jobs and add turbo cache

[h4yfans]

What

Refactor the CI Lint, Typecheck, and Tests workflow to run quality checks in parallel and cache Turbo artifacts, cutting wall-clock CI time from ~12-15 min to ~6-7 min cold (~2-3 min warm).

Why

The single quality job serialized boundary checks → lint → typecheck → tests on one runner. GitHub Actions bills by minutes, not jobs, so fanning out into parallel jobs costs the same minutes but collapses wall clock. Without a Turbo cache, every PR also rebuilt packages that never changed.

How

• 4 parallel jobs (boundaries, lint, typecheck, test) replace the single quality job. Wall clock is now bounded by the slowest job.
• Turbo cache via actions/cache@v4 on .turbo/ with TURBO_CACHE_DIR env var and restore-keys fallback. Turbo's content hashing then skips unchanged packages.
• --ignore-scripts on lint/typecheck/boundaries jobs skips better-sqlite3/keytar native rebuilds (~30-60s each). Only the test job needs native bindings, so libsecret-1-dev apt install stays there.
• Boundary checks (check:contracts, check:architecture, ipc:check) run with & + wait instead of sequentially.
• paths-ignore skips the entire workflow for docs-only PRs (**/*.md, docs/**, LICENSE).

Type

• ci — CI/CD changes

Test plan

• This PR's own CI run validates the new workflow end-to-end
• Confirm all 4 jobs pass in parallel
• Confirm Turbo cache hit on a follow-up no-op commit (warm-cache path)
• Confirm a docs-only commit (e.g. README tweak) correctly skips the workflow

Checklist

• Self-reviewed the diff
• No hardcoded secrets or credentials
• Files stay under ~500 LOC
• Follows immutable data patterns