This implements the ability to separate bd-specific instructions from
project-specific instructions by generating a canonical BD_GUIDE.md file.
## Changes
1. Added `--output` flag to `bd onboard` command
- Generates version-stamped BD_GUIDE.md at specified path
- Includes both agentsContent and copilotInstructionsContent
- Auto-generated header warns against manual editing
2. Version tracking integration
- checkAndSuggestBDGuideUpdate() detects outdated BD_GUIDE.md
- Suggests regeneration when bd version changes
- Integrated with maybeShowUpgradeNotification()
3. Comprehensive test coverage
- Tests for BD_GUIDE.md generation
- Tests for version stamp validation
- Tests for content inclusion
4. Documentation updates
- Updated AGENTS.md with BD_GUIDE.md workflow
- Added regeneration instructions to upgrade workflow
## Benefits
- Clear separation of concerns (bd vs project instructions)
- Deterministic updates (no LLM involved)
- Git-trackable diffs show exactly what changed
- Progressive disclosure (agents read when needed)
## Usage
\`\`\`bash
# Generate BD_GUIDE.md
bd onboard --output .beads/BD_GUIDE.md
# After upgrading bd
bd onboard --output .beads/BD_GUIDE.md # Regenerate
\`\`\`
Closes bd-woro
