chore: remove obsolete BD_GUIDE.md references

BD_GUIDE.md generation was removed in 125e36d5 when bd onboard was
simplified to output a minimal snippet pointing to bd prime.

This commit:
- Removes stale BD_GUIDE.md references from CLAUDE.md
- Removes dead checkAndSuggestBDGuideUpdate() code from version_tracking.go
- Removes BD_GUIDE.md from UNINSTALLING.md file list

Closes #660 (already fixed in v0.30.7 via f1380e8b)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/UNINSTALLING.md

@@ -111,7 +111,6 @@ The `.beads/` directory contains:
 | `config.yaml` | Project configuration |
 | `metadata.json` | Version tracking |
 | `deletions.jsonl` | Soft-deleted issues |
-| `BD_GUIDE.md` | AI agent instructions |
 | `README.md` | Human-readable overview |
 
 Remove everything: