feat: Add version compatibility checking to plugin

Adds version management to help users keep bd CLI and plugin in sync.

Changes:
- Add engines field to plugin.json requiring bd >=0.9.0
- Add /bd-version command to check component versions
- Add comprehensive "Updating" section to PLUGIN.md
- Document recommended update workflow

Users can now run /bd-version to check:
- bd CLI version
- Plugin version
- MCP server status
- Compatibility warnings

Addresses version sync concerns raised in plugin development.

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

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

.claude-plugin/plugin.json

@@ -19,6 +19,9 @@
     "agent-memory",
     "mcp-server"
   ],
+  "engines": {
+    "beads": ">=0.9.0"
+  },
   "mcpServers": {
     "beads": {
       "command": "uv",