refactor: move Claude Code plugin to dedicated subdirectory (GH#985)

Move all plugin content to claude-plugin/ subdirectory for cleaner
separation from core beads functionality:

- claude-plugin/.claude-plugin/plugin.json - plugin manifest
- claude-plugin/commands/ - all slash commands
- claude-plugin/agents/ - task-agent (now in correct location)
- claude-plugin/skills/beads/ - beads skill with resources

Root .claude-plugin/ now only contains marketplace.json pointing
to ./claude-plugin.

Note: After updating, users may need to clear plugin cache:
  rm -rf ~/.claude/plugins/cache/beads-marketplace/

Fixes #985

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Executed-By: beads/crew/dave
Rig: beads
Role: crew

claude-plugin/commands/version.md

@@ -0,0 +1,26 @@
+---
+description: Check beads and plugin versions
+---
+
+Check the installed versions of beads components and verify compatibility.
+
+**Note:** The MCP server automatically checks bd CLI version >= 0.9.0 on startup. This command provides detailed version info and update instructions.
+
+Use the beads MCP tools to:
+1. Run `bd version` via bash to get the CLI version
+2. Check the plugin version (0.9.2)
+3. Compare versions and report any mismatches
+
+Display:
+- bd CLI version (from `bd version`)
+- Plugin version (0.9.2)
+- MCP server version (0.9.2)
+- 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/scripts/install.sh | bash`
+- Update plugin: `/plugin update beads`
+- Restart Claude Code after updating
+
+Suggest checking for updates if the user is on an older version.