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-update.md

@@ -0,0 +1,22 @@
+---
+description: Update an issue's status, priority, or other fields
+argument-hint: [issue-id] [status]
+---
+
+Update a beads issue.
+
+If arguments are provided:
+- $1: Issue ID
+- $2: New status (open, in_progress, blocked, closed)
+
+If arguments are missing, ask the user for:
+1. Issue ID
+2. What to update (status, priority, assignee, title, description)
+3. New value
+
+Use the beads MCP `update` tool to apply the changes. Show the updated issue to confirm the change.
+
+Common workflows:
+- Start work: Update status to `in_progress`
+- Mark blocked: Update status to `blocked`
+- Reprioritize: Update priority (0-4)