feat: Add agent upgrade awareness documentation and tooling

Implements bd-hwmp and bd-5otr (part of epic bd-nxgk) to help AI agents
automatically detect and adapt to bd upgrades.

## Changes

### Documentation (bd-hwmp)
- Added "After Upgrading bd" section to AGENTS.md with step-by-step workflow
- Enhanced "Pro Tips for Agents" with upgrade detection guidance
- Documents bd info --whats-new and bd hooks install commands
- References GitHub Discussion #239

### Startup Hook (bd-5otr)
- Created examples/startup-hooks/bd-version-check.sh
  - Auto-detects bd version changes via .beads/metadata.json
  - Shows bd info --whats-new when upgrade detected
  - Auto-updates outdated git hooks
  - Handles edge cases (no jq, not in beads project, etc.)
- Created examples/startup-hooks/README.md with integration examples
  - Claude Code, GitHub Copilot, Cursor integration patterns
  - Comprehensive usage and troubleshooting guide
- Updated examples/README.md with new startup-hooks section

## Impact

Agents can now:
- Automatically detect bd upgrades at session start
- See what changed without re-reading all documentation
- Keep git hooks in sync automatically
- Adapt workflows based on new features

Works today with zero bd code changes required!

Related: #239

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.beads/metadata.json

@@ -1,4 +1,5 @@
 {
   "database": "beads.db",
-  "jsonl_export": "beads.jsonl"
-}
+  "jsonl_export": "beads.jsonl",
+  "last_bd_version": "0.24.2"
+}