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-create.md

@@ -0,0 +1,19 @@
+---
+description: Create a new issue interactively
+argument-hint: [title] [type] [priority]
+---
+
+Create a new beads issue. If arguments are provided:
+- $1: Issue title
+- $2: Issue type (bug, feature, task, epic, chore)
+- $3: Priority (0-4, where 0=critical, 4=backlog)
+
+If arguments are missing, ask the user for:
+1. Issue title (required)
+2. Issue type (default: task)
+3. Priority (default: 2)
+4. Description (optional)
+
+Use the beads MCP `create` tool to create the issue. Show the created issue ID and details to the user.
+
+Optionally ask if this issue should be linked to another issue (discovered-from, blocks, parent-child, related).