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/commands/bd-version.md

@@ -0,0 +1,22 @@
+---
+description: Check beads and plugin versions
+---
+
+Check the installed versions of beads components and verify compatibility.
+
+Use the beads MCP tools to:
+1. Run `bd --version` via bash to get the CLI version
+2. Check the plugin version from the environment
+3. Compare versions and report any mismatches
+
+Display:
+- bd CLI version (from `bd --version`)
+- Plugin version (0.9.0)
+- MCP server status (from `stats` tool or connection test)
+- Compatibility status (✓ compatible or ⚠️ update needed)
+
+If versions are mismatched, provide instructions:
+- Update bd CLI: `curl -fsSL https://raw.githubusercontent.com/steveyegge/beads/main/install.sh | bash`
+- Update plugin: `/plugin update beads`
+
+Suggest checking for updates if the user is on an older version.