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>

README.md

@@ -138,9 +138,6 @@ Skills we'd like to see:
 **Communication Channels**
 - `/add-signal` - Add Signal as a channel
 
-**Session Management**
-- `/clear` - Add a `/clear` command that compacts the conversation (summarizes context while preserving critical information in the same session). Requires figuring out how to trigger compaction programmatically via the Claude Agent SDK.
-
 ## Requirements
 
 - macOS or Linux