refactor: rename levelup.spec to levelup.specify and consolidate extension docs

[kanfil]

Summary

Two improvements in this PR:

1. Rename levelup.spec → levelup.specify

Renames the command for consistency with architect.specify and full-verb naming convention across all extensions.

Changes:

• Renamed extensions/levelup/commands/spec.md → specify.md
• Updated extension.yml with new command name
• Updated all 53 internal references across codebase
• Updated documentation (README, docs, tests, catalog)
• Bumped version to 0.0.117

2. Move extension documentation to extension READMEs

Consolidates extension documentation into their respective READMEs, reducing duplication in main README.

Changes:

• Moved architecture documentation from main README to extensions/architect/README.md
• Added Two-Level Architecture System, configuration options (--views, --adr-heuristic) to architect README
• Replaced main README architecture section with brief summary + link to extension README
• Added link to levelup README in main README
• Reorganized slash commands table with extension sections and links

Benefits:

• Each extension is now self-documented
• Main README is more concise
• Easier to maintain extension-specific docs
• Clear separation between core and extension commands

Test Results

Automated Tests ✅

All tests passing:

Trace Command Tests (12/12 passed):

• ✅ Trace generation and validation scripts exist
• ✅ Command template structure validated
• ✅ LevelUp integration confirmed
• ✅ Summary section validation in all templates

Extension Tests (45/45 passed):

• ✅ Extension manifest validation
• ✅ Extension registry and manager tests
• ✅ Command registrar tests (all agents including kiro-cli)
• ✅ Extension catalog tests
• ✅ Integration tests

Manual Verification ✅

Command rename verification:

• ✅ No remaining levelup.spec references in codebase (except CHANGELOG)
• ✅ All internal references updated to levelup.specify
• ✅ Extension.yml correctly configured with new command name
• ✅ Test file updated to reference specify.md

Documentation verification:

• ✅ Main README architecture section replaced with summary
• ✅ Architect README contains all documentation from main README
• ✅ LevelUp README link added to main README
• ✅ Slash commands table reorganized with extension sections
• ✅ All extension README links resolve correctly

Files changed: 18 files

• 12 documentation files updated
• 1 command file renamed
• 5 configuration files updated
• Version bumped to 0.0.117

Test Plan Checklist

• Verify /levelup.specify command works (no backward compatibility)
• Verify extension READMEs have complete documentation
• Verify main README links resolve correctly
• Verify slash commands table is accurate
• All automated tests passing
• No remaining old references in codebase

Breaking Change Notice

⚠️ Breaking Change: /levelup.spec is no longer available. Users must use /levelup.specify.

No backward compatibility alias provided as per project decision.