docs: Mutation-Gated Testing — demonstrable killer per #676

[Hero-Gamer]

Closes #676.

Docs-only change — zero production Rust touched. 3 files, purely additive:

• AGENTS.md — hard rules
• TESTING.md — Tier definitions, first-commit gate, survivor classification
• .agents/skills/ship-pr/SKILL.md — pre-submission checklist

Tier5 (docs-only) → self-exempt. No new CI job, no runtime change.

Natural follow-ups if this lands clean:

• ast-grep rules to enforce exact variants & deterministic time
• Baseline-diff mutant feedback making existing advisory output actionable