feat: implement RFC 0006 (soft-delete) and RFC 0007 (branch naming)
RFC 0006 - Document Deletion Tools:
- Add soft-delete with 7-day retention before permanent deletion
- Add blue_delete, blue_restore, blue_deleted_list, blue_purge_deleted tools
- Add deleted_at column to documents table (schema v3)
- Block deletion of documents with ADR dependents
- Support dry_run, force, and permanent options
RFC 0007 - Consistent Branch Naming:
- Strip RFC number prefix from branch/worktree names
- Branch format: feature-description (not rfc/NNNN-feature-description)
- PR title format: RFC NNNN: Feature Description
- Add strip_rfc_number_prefix helper with tests
Also:
- Remove orphan .blue/repos/ and .blue/data/ directories
- Fix docs path resolution bug (spike documented)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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