fix: pnpm test ship-gate meta-script

[bugsyhewitt]

Summary

• Adds missing test script to package.json that runs the full ship-gate suite (vitest + pytest).
• Fixes CONTRIBUTING.md line 73 which referenced non-existent pnpm build and pnpm check scripts.
• CLAUDE.md is intentionally not touched (per overmind directive — 42-line file, no ship-gate text).

Files changed

• package.json — adds test script: pnpm exec vitest run && PYTHONPATH=src pytest
• CONTRIBUTING.md — replaces broken test command with truthful pnpm test + measured counts

Ship gate

pnpm test exit 0 on fresh origin/main base (c0f56a8):

• vitest: 430 passed, 34 skipped (29/30 test files passed, 1 skipped)
• pytest: 756 passed, 125 skipped
• combined: 1,186 passed, 159 skipped

Context

Closes the gap flagged in the packet-37.1 audit: pnpm test is the documented ship gate but was not actually wired up. pnpm build and pnpm check are not defined scripts on main, so the previous CONTRIBUTING.md instruction was unrunnable.

Replaces PR #19 (closed — that was 459 files off a stale base).