docs: add CONTRIBUTING with translations policy

Translation PRs (like #13) are declined as policy: fast-moving docs make
committed translations stale within days, and one accepted language obliges
maintaining several. On-demand translation serves readers better. Also codifies
issue-first for non-trivial PRs, Conventional Commits, and the just-check gate.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CONTRIBUTING.md

@@ -0,0 +1,28 @@
+# Contributing
+
+Issues and PRs are welcome. A few ground rules keep them mergeable.
+
+## Before you open a PR
+
+- **Open an issue first** for anything non-trivial (new skills, behaviour
+  changes). Small fixes and typo-level docs can go straight to a PR.
+- Commits follow [Conventional Commits](https://www.conventionalcommits.org/):
+  `type(scope): summary` - see `rules/commit-style.md` for the house specifics.
+- Run the gate before pushing: `just check` (frontmatter validation, doc-drift,
+  resource contracts, and every skill's behavioural suite).
+- Conventions for skills, agents, and naming live in
+  [AGENTS.md](AGENTS.md) and `docs/SKILL-CREATION-PROTOCOL.md` - read those
+  before adding components.
+
+## What isn't accepted
+
+- **Translations.** README and docs change too frequently for committed copies
+  to stay current - a translated README is stale within days, and one language
+  fairly obliges maintaining several. On-demand browser/AI translation serves
+  readers better than a rotting copy. Translation PRs are closed with thanks.
+- Drive-by dependency bumps outside the bot's cadence, cosmetic reformatting,
+  and generated-file edits (anything marked "built - don't hand-edit").
+
+## Licence
+
+Contributions are accepted under the repository's MIT licence.