chore: add test-audit skill adapted from openclaw for all agents

[Tryanks]

Adds a test-audit skill under .agents/skills/test-audit/, adapted from OpenClaw's test-audit at upstream 80930af, and makes CONTRIBUTING ask for restraint when tests are written.

• SKILL.md / CAMPAIGN.md keep upstream's authoring gate, audit and campaign workflow, and defer to CONTRIBUTING's Tests earn their maintenance as the value bar. Vitest/TypeScript tooling, OpenClaw paths and skills, and the Telegram campaign examples are replaced with Cargo/nextest, this workspace's crates, Rust test-only seams (test-support features, #[cfg(test)] accessors), and lessons from Audit test contracts, stabilize fixtures, and fix uncovered defects #487. The skill's description (its trigger) is unchanged from upstream.
• README.md links the upstream source and pinned commit, notes what was adapted and how to sync, and carries the upstream MIT notice.
• .claude/skills is a symlink to ../.agents/skills, so Claude Code loads the same skills as Codex, OpenCode, Cursor and other agents that read .agents/skills/.
• CONTRIBUTING → Review → Tests earn their maintenance now opens with weighing a test before writing it (name the contract or failure, check existing coverage, extend a case before adding a near-duplicate), rules out seams only a test uses, and points to the skill when a test's value is in doubt or when auditing/pruning. Principles are unchanged.

No code changes; only CI's standard checks apply.