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/commands/bd-ready.md

@@ -0,0 +1,15 @@
+---
+description: Find ready-to-work tasks with no blockers
+---
+
+Use the beads MCP server to find tasks that are ready to work on (no blocking dependencies).
+
+Call the `ready` tool to get a list of unblocked issues. Then present them to the user in a clear format showing:
+- Issue ID
+- Title
+- Priority
+- Issue type
+
+If there are ready tasks, ask the user which one they'd like to work on. If they choose one, use the `update` tool to set its status to `in_progress`.
+
+If there are no ready tasks, suggest checking `blocked` issues or creating a new issue with the `create` tool.