Add CONTRIBUTING.md: collaboration guidelines for humans and AI agents

CONTRIBUTING.md

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+# Contributing to Lunar Pup
+
+This is a fully collaborative project — humans and AI agents working in parallel. These
+rules exist so nobody steps on anyone's toes. They apply to everyone, human or agent.
+
+## The two golden rules
+
+1. **Check open issues and PRs before filing anything.**
+   ```bash
+   gh pr list --repo Tsurgcom/lunarpup --state open
+   gh issue list --repo Tsurgcom/lunarpup --state open
+   ```
+   If something overlapping exists, comment there instead of opening a duplicate.
+
+2. **Open a PR for what you plan to work on — before or as you start, not when you
+   finish.** A draft PR with a clear title, the files/areas you expect to touch, and a
+   sentence on the approach is enough. This is how everyone else finds out what's claimed.
+   Big ideas floating in chat (e.g. a renderer rewrite) don't exist until they have an
+   issue or draft PR — declare them.
+
+## Declaring scope
+
+- In your PR body, list the files/directories you expect to touch (a `TOUCHES:` line).
+  Overlap with another open PR? Talk to its author *before* both of you sink time in.
+- Roadmap work is tracked in `plans/lunar-pup-vision/` — each concern doc has
+  `TOUCHES` and `BLOCKED_BY`. Link the concern your PR implements. New large work
+  deserves its own plan doc in the same format.
+- Keep PRs to one landable unit. A PR that rewrites the renderer *and* adds a gamemode
+  can't be reviewed or sequenced against anyone else's work.
+
+## Quality gate
+
+Every PR must pass before review is requested:
+
+```bash
+bun install && bunx tsc --noEmit && bun test
+```
+
+Add tests for what you build. If your change has runtime behavior, say in the PR body how
+you verified it live (not just typecheck).
+
+## Preview your PR before marking it ready
+
+Every PR must be testable in a browser without the reviewer building it locally. Before
+you flip a draft to **ready for review**, deploy a preview of your branch to your own
+Netlify and put the URL in the PR description.
+
+Zero-setup (anonymous draft deploy — no account needed):
+
+```bash
+bun run build
+npx netlify-cli deploy --dir=dist --allow-anonymous
+```
+
+Or with your own Netlify account (one-time `npx netlify-cli login` + `npx netlify-cli init`,
+then per-PR):
+
+```bash
+npx netlify-cli deploy --build
+```
+
+Both print a unique draft URL — that's your PR preview. Details in `NETLIFY.md`.
+
+Note: Netlify only hosts the static game. The Bun WebSocket server doesn't run there, so
+multiplayer on a preview needs `?ws=<your-server>` pointed at a reachable server (or state
+that your PR is frontend-only).
+
+## Overlapping files are normal — clobbering is not
+
+Two PRs touching the same file is fine when coordinated: agree on merge order in PR
+comments, and the later one rebases. Never resolve a conflict by discarding the other
+side's behavior — combine both, and say so in the PR.
+
+## AI-agent etiquette
+
+Agents follow every rule above, plus: identify yourself in the PR body (which
+harness/fleet, which plan concern), include your acceptance gate command, and never
+force-push over a branch you didn't create.