docs: update contributing guidelines and skill type taxonomy

- Rewrite CONTRIBUTING.md with four skill types (feature, utility,
  operational, container), PR requirements, pre-submission checklist
- Update PR template with skill type checkboxes and docs option
- Add label-pr workflow to auto-label PRs from template checkboxes
- Add hidden template version marker (v1) for follows-guidelines label
- Update CLAUDE.md with skill types overview and contributing instruction
- Update skills-as-branches.md to reference full taxonomy
- Remove /clear from README RFS (already exists as /add-compact)
- Delete obsolete docs (nanorepo-architecture.md, nanoclaw-architecture-final.md)

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

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,14 +1,18 @@
+<!-- contributing-guide: v1 -->
 ## Type of Change
 
-- [ ] **Skill** - adds a new skill in `.claude/skills/`
+- [ ] **Feature skill** - adds a channel or integration (source code changes + SKILL.md)
+- [ ] **Utility skill** - adds a standalone tool (code files in `.claude/skills/<name>/`, no source changes)
+- [ ] **Operational/container skill** - adds a workflow or agent skill (SKILL.md only, no source changes)
 - [ ] **Fix** - bug fix or security fix to source code
 - [ ] **Simplification** - reduces or simplifies source code
+- [ ] **Documentation** - docs, README, or CONTRIBUTING changes only
 
 ## Description
 
 
 ## For Skills
 
-- [ ] I have not made any changes to source code
-- [ ] My skill contains instructions for Claude to follow (not pre-built code)
+- [ ] SKILL.md contains instructions, not inline code (code goes in separate files)
+- [ ] SKILL.md is under 500 lines
 - [ ] I tested this skill on a fresh clone