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-stats.md

@@ -0,0 +1,17 @@
+---
+description: Show project statistics and progress
+---
+
+Display statistics about the current beads project.
+
+Use the beads MCP `stats` tool to retrieve project metrics and present them clearly:
+- Total issues by status (open, in_progress, blocked, closed)
+- Issues by priority level
+- Issues by type (bug, feature, task, epic, chore)
+- Completion rate
+- Recently updated issues
+
+Optionally suggest actions based on the stats:
+- High number of blocked issues? Run `/bd-blocked` to investigate
+- No in-progress work? Run `/bd-ready` to find tasks
+- Many open issues? Consider prioritizing with `/bd-update`