Make AGENTS.md canonical and reach every runtime (stack 3/5)

[martient]

Stacked on #43. Resolves P1-8, P1-9, P1-10, P1-11 and P2-14.

One instruction file, not two

AGENTS.md and CLAUDE.md were near-identical files kept in sync by hand — a two-line diff that had already started drifting: the AI-attribution-trailer ban landed in CLAUDE.md only. AGENTS.md is now the single source; CLAUDE.md is an @AGENTS.md import plus the one genuinely Claude-specific line (slash-command invocation), which is the pattern the ecosystem settled on.

Two widely-deployed runtimes read neither file, so they get thin pointers: .github/copilot-instructions.md and GEMINI.md.

Cross-tool skill discovery

.agents/skills is the path Codex CLI, the Codex app, Claude Code and Copilot CLI all resolve. It now exists as a symlink to the canonical plugins/committy/skills, so there is no second copy to drift.

⚠️ Reviewer note on the symlink. A checkout that materialises it as a plain text file — Windows without developer mode, or core.symlinks=false — silently breaks discovery for every runtime reading that path. test_skills_are_discoverable_at_the_cross_tool_path fails loudly in that case rather than letting it pass unnoticed. If you'd rather not depend on symlinks at all, the alternative is moving the canonical copy to .agents/skills and pointing both plugin manifests there; I didn't take it because I can't verify from here whether Claude's plugin.json accepts a skills path outside the plugin directory.

Skill frontmatter

All four skills carried only name and description. They now add the spec's optional fields:

• license: Apache-2.0
• allowed-tools — so security-conscious runtimes pre-authorise exactly what each skill needs (committy-enforce is the only one granted Edit/Write, since it installs hooks and CI)
• metadata — a skill version independent of the CLI version, plus the minimum Committy it expects

Codex syntax in a shared body

committy-release/SKILL.md referenced siblings as $committy-commit. That body is shared by both runtimes, and Claude Code invokes them as /committy:committy-commit. Siblings are now named plainly and each runtime resolves them its own way.

Test contract change

test_agent_docs_mention_the_non_interactive_env_var previously required every agent-facing file to contain COMMITTY_NONINTERACTIVE verbatim. With one canonical file that is wrong, so it now accepts a file that either states the rule or defers to AGENTS.md — and covers the two new pointer files too.

Verification

375 passed, fmt clean, clippy -D warnings clean. The pre-existing tag_tests failure noted in #42 is unchanged.

[martient]

Superseded by #49, which carries these commits unchanged. Closing — see #42 for why the stack was collapsed.

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