Epic: Hygiene baseline alignment — gates, docs, taxonomy

[lbliii]

From the 2026-07-08 cross-stack hygiene survey. Rosettes has the stack's best golden-corpus practice (55-language token fixtures with a CI job that polices coverage/tiers/completeness) and the cleanest ty config (zero downgrades) — but the weakest everything else. Canonical baseline doc lands in pounce docs/design/stack-hygiene-baseline.md.

Tasks

• Coverage gate in CI (currently none; use patitas' documented honest-ratchet pattern if below 80)
• Benchmark job in CI with regression gate (see the companion evidence epic)
• Create docs/ (architecture notes, troubleshooting) + CONTRIBUTING.md + SECURITY.md + AGENTS.md
• Issue label taxonomy: saga/epic/task + priority + type + theme (pounce scheme; this repo had default labels only)
• Bump CI action pins (checkout@v4 / setup-uv@v4 are the oldest in the stack)
• Plain-3.14 (GIL) CI lane alongside 3.14t
• Port zoomies' silent-exception + raise-message CI gate

Acceptance

CI enforces coverage + benchmark gates on both interpreter lanes; governance docs exist; taxonomy matches the stack scheme.