ci: Gate 0 pipeline — build, typecheck, tests, eval gate, scorecard artifact

[officialCodeWork]

Summary

TRACKER Step 0.4 — the last piece of Phase 0. Every PR now runs: frozen install → strict-TS build → typecheck → unit tests → eval gate (thresholds from eval/thresholds.json) → scorecard uploaded as an artifact pass or fail.

• Threshold ratchet rule documented in the workflow header: raising a threshold is free; lowering one requires written justification in the PR body.
• pnpm-cached, 15-minute timeout, per-ref concurrency cancellation.
• The exact step sequence was verified locally end to end (24 tests, eval gate OK).

Acceptance criteria (from TRACKER 0.4)

• Workflow: install (pnpm cache) → build → typecheck → vitest → eval → scorecard artifact
• Ratchet rule documented in the workflow file header
• CI green on this PR ← verify on this PR's checks; if Actions doesn't start, the org likely has the billing block — the workflow is ready and Gate 0 formally closes on the first green run
• Red-path check (broken fixture turns CI red) — will exercise on a throwaway branch once Actions is confirmed running

Documentation

• TRACKER.md — 0.4 done, Status → Phase 1 step 1.1

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