feat: add CONTRIBUTING.md, GitHub Issue templates, contrib:open label convention

CONTRIBUTING.md:
- Two tracks: humans (GitHub flow) and agents (beadboard-driver + bd)
- Copy-pasteable agent instructions for quick setup
- PR size guidelines (under 100 lines preferred, 200+ needs issue first)
- Quality gates, claim workflow, professional conduct expectations
- beadboard-driver and bd setup for agent contributors

GitHub Issue templates:
- Bug report: structured fields (steps, platform, Dolt status, versions)
- Feature request: problem/proposal/area, opt-in to implement
- Config: links to Discord and contrib:open issue filter

contrib:open labels:
- Tagged 4 orchestrator beads for community contribution
- Left session race condition (beadboard-4v7) as internal

README:
- Updated banner to link to CONTRIBUTING.md
- Updated Contributing section with Discord link and contrib:open workflow

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

README.md

@@ -1,4 +1,4 @@
-## 🚧 Orchestrator is working but new — use your own coding agent for now (unless you want to help!) 🚧
+## 🚧 The bb-pi orchestrator is under construction — [help us build it!](CONTRIBUTING.md) Use your own coding agent alongside BeadBoard for now. 🚧
 
 # BeadBoard
 
@@ -348,15 +348,16 @@ npm run test         # Full test suite
 
 ## 🤝 Contributing
 
-1. Fork the repository
-2. Create a feature branch: `git checkout -b feature/my-feature`
-3. Run quality gates before submitting:
+We welcome contributions from humans and AI agents. See **[CONTRIBUTING.md](CONTRIBUTING.md)** for the full guide.
 
-```bash
-npm run typecheck && npm run lint && npm run test
-```
+Quick version:
+1. Find work: check [GitHub Issues](https://github.com/zenchantlive/beadboard/issues) or run `bd list --label contrib:open`
+2. Small PRs preferred (under ~100 lines). For larger changes, open an issue first.
+3. Run the gates: `npm run typecheck && npm run lint && npm run test`
+4. PR against `main`
 
-4. Submit a pull request against `main`
+<!-- TODO: replace with actual Discord invite link -->
+**[Join the Discord](https://discord.gg/YOUR_INVITE)** to discuss contributions, get help, or coordinate on larger work.
 
 ---