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>

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,9 @@
+blank_issues_enabled: true
+contact_links:
+  - name: Discord
+    # TODO: replace with actual Discord invite link
+    url: https://discord.gg/YOUR_INVITE
+    about: Chat with the community, coordinate on contributions, get help
+  - name: Available work
+    url: https://github.com/zenchantlive/beadboard/issues?q=is%3Aissue+is%3Aopen+label%3Acontrib%3Aopen
+    about: Browse issues tagged for community contribution