feat: Add Claude Code plugin for beads [addresses #28]

Adds a Claude Code plugin for one-command installation of beads via
/plugin command. The plugin bundles the MCP server, slash commands,
and an autonomous task agent.

Components:
- Plugin metadata with MCP server configuration
- 8 slash commands for core workflow (/bd-ready, /bd-create, etc.)
- Task agent for autonomous execution (@task-agent)
- Comprehensive plugin documentation (PLUGIN.md)

The plugin provides a lower-friction installation path for Claude Code
users who want integrated slash commands rather than direct MCP tools.

Related: https://github.com/steveyegge/beads/issues/28

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

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

.claude-plugin/marketplace.json

@@ -0,0 +1,15 @@
+{
+  "name": "beads-marketplace",
+  "description": "Local marketplace for beads plugin development",
+  "owner": {
+    "name": "Steve Yegge"
+  },
+  "plugins": [
+    {
+      "name": "beads",
+      "source": ".",
+      "description": "AI-supervised issue tracker for coding workflows",
+      "version": "0.9.0"
+    }
+  ]
+}