docs: update README and ROADMAP for Firefox extension project

Updated documentation to reflect current project state:

README.adoc:
- Changed from feature flag management to Firefox flag management
- Added installation and usage instructions
- Current status table (99% complete)
- Flag database breakdown (105 flags, 8 categories)
- Privacy & security section
- Development guide
- Build system documentation

ROADMAP.adoc:
- v0.1.0 status (ready for submission)
- Version history and release notes
- v0.2.0-v1.0.0 roadmap
- Long-term vision (v2.0-v3.0)
- Community contribution guidelines
- Release schedule and success metrics

Both files now accurately reflect FireFlag as a Firefox/Gecko flag
management extension ready for Mozilla Add-ons submission.

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

Author: Jonathan D.A. Jewell