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-close.md

@@ -0,0 +1,18 @@
+---
+description: Close a completed issue
+argument-hint: [issue-id] [reason]
+---
+
+Close a beads issue that's been completed.
+
+If arguments are provided:
+- $1: Issue ID
+- $2+: Completion reason (optional)
+
+If the issue ID is missing, ask for it. Optionally ask for a reason describing what was done.
+
+Use the beads MCP `close` tool to close the issue. Show confirmation with the issue details.
+
+After closing, suggest checking for:
+- Dependent issues that might now be unblocked (use `ready` tool)
+- New work discovered during this task (use `create` tool with `discovered-from` link)