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-show.md

@@ -0,0 +1,17 @@
+---
+description: Show detailed information about an issue
+argument-hint: [issue-id]
+---
+
+Display detailed information about a beads issue.
+
+If an issue ID is provided as $1, use it. Otherwise, ask the user for the issue ID.
+
+Use the beads MCP `show` tool to retrieve issue details and present them clearly, including:
+- Issue ID, title, and description
+- Status, priority, and type
+- Creation and update timestamps
+- Dependencies (what this issue blocks or is blocked by)
+- Related issues
+
+If the issue has dependencies, offer to show the full dependency tree.