docs: update all .md files to reflect current project status
Updated 4 markdown files with current state:
EXTENSION_STATUS.md:
- Changed status from DEBUGGING to COMPLETE
- Updated decision graph count (295 → 332 nodes)
- Added recently completed section (nodes 296-332)
- Marked all extension bugs as resolved
CONTRIBUTING.md:
- Replaced npm with pnpm throughout
- Added monorepo structure documentation
- Updated development commands (netlify-cli dev --filter)
- Added extension development workflow
PLAN.md:
- Updated status to Phase 1 COMPLETE
- Added all recent fixes to completion list
- Updated decision graph count to 332 nodes
- Added changelog entries for latest work
packages/extension/README.md:
- Added prerequisites section (dev server + login required)
- Updated build commands with dev/prod distinction
- Added Step 0: Start ATlast Dev Server
- Added common issues for auth and server states
All files now accurately reflect completion status and use pnpm.