Implement BD_GUIDE.md generation for version-stamped documentation (bd-woro)

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   - checkAndSuggestBDGuideUpdate() detects outdated BD_GUIDE.md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

AGENTS.md

@@ -38,12 +38,17 @@ bd info --whats-new
 # 2. Update git hooks to match new bd version
 bd hooks install
 
-# 3. Check for any outdated hooks (optional)
+# 3. Regenerate BD_GUIDE.md if it exists (optional but recommended)
+bd onboard --output .beads/BD_GUIDE.md
+
+# 4. Check for any outdated hooks (optional)
 bd info  # Shows warnings if hooks are outdated
 ```
 
 **Why update hooks?** Git hooks (pre-commit, post-merge, pre-push) are versioned with bd. Outdated hooks may miss new auto-sync features or bug fixes. Running `bd hooks install` ensures hooks match your bd version.
 
+**About BD_GUIDE.md:** This is an optional auto-generated file that separates bd-specific instructions from project-specific ones. If your project uses this file (in `.beads/BD_GUIDE.md`), regenerate it after upgrades to get the latest bd documentation. The file is version-stamped and should never be manually edited.
+
 **Related:** See GitHub Discussion #239 for background on agent upgrade workflows.
 
 ## Human Setup vs Agent Usage