refactor: move Claude Code plugin to dedicated subdirectory (GH#985)

Move all plugin content to claude-plugin/ subdirectory for cleaner
separation from core beads functionality:

- claude-plugin/.claude-plugin/plugin.json - plugin manifest
- claude-plugin/commands/ - all slash commands
- claude-plugin/agents/ - task-agent (now in correct location)
- claude-plugin/skills/beads/ - beads skill with resources

Root .claude-plugin/ now only contains marketplace.json pointing
to ./claude-plugin.

Note: After updating, users may need to clear plugin cache:
  rm -rf ~/.claude/plugins/cache/beads-marketplace/

Fixes #985

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Executed-By: beads/crew/dave
Rig: beads
Role: crew

claude-plugin/commands/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 `/beads:blocked` to investigate
+- No in_progress work? Run `/beads:ready` to find tasks
+- Many open issues? Consider prioritizing with `/beads:update`