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/blocked.md

@@ -0,0 +1,15 @@
+---
+description: Show blocked issues
+argument-hint: []
+---
+
+Show all issues that are blocked by dependencies.
+
+Use `bd blocked` to see which issues have blockers preventing them from being worked on. This is the inverse of `bd ready` - it shows what's NOT ready.
+
+Blocked issues have one or more dependencies with type "blocks" that are still open. Once all blocking dependencies are closed, the issue becomes ready and will appear in `bd ready`.
+
+Useful for:
+- Understanding why work is stuck
+- Identifying critical path items
+- Planning dependency resolution