feat(grasp-05): implement archive mode for backup/mirror operation

Implements GRASP-05 specification for accepting repository announcements
that don't list this relay, enabling archive, mirror, and backup use cases.

Core Features:
- Three whitelist formats: <npub>, <npub>/<identifier>, <identifier>
- Archive-all mode for complete ecosystem mirrors
- Fail-fast npub validation at startup
- Read-only enforcement (archived repos reject pushes)
- Full GRASP-02 sync (git data + Nostr events)
- Dynamic archive status (no flags/metadata)

Implementation:
- Add ArchiveWhitelistEntry enum with Pubkey/Repository/Identifier variants
- Add ArchiveConfig with validation and matching logic
- Update AnnouncementResult to include AcceptArchive variant
- Refactor validate_announcement() to return AnnouncementResult with archive check
- Update AnnouncementPolicy with catch-all pattern for cleaner code
- Wire archive config through builder and policy layers

Configuration:
- NGIT_ARCHIVE_ALL: Accept all announcements (⚠️ storage risk)
- NGIT_ARCHIVE_WHITELIST: Comma-separated whitelist entries
- Updated docs, .env.example, and nix/module.nix

Testing:
- 28 unit tests for config parsing and whitelist matching
- 7 integration tests for archive mode validation
- All 296 tests passing

Validation Priority:
1. Lists our service → Accept (GRASP-01, read/write)
2. Is maintainer → AcceptMaintainer (multi-maintainer, read/write)
3. Matches archive config → AcceptArchive (GRASP-05, read-only)
4. None of above → Reject

Security Considerations:
- Archive-all mode has storage/bandwidth DoS risk
- Identifier-only format matches any pubkey (use npub/identifier for high-value)
- Invalid npubs cause startup failure (fail-fast)

Documentation:
- Concise explanation focused on rationale
- Reference docs updated with all config options
- README updated to reflect completed feature
- Removed from roadmap, added to compliance section

See docs/explanation/grasp-05-archive.md for details.

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