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/epic.md

@@ -0,0 +1,26 @@
+---
+description: Epic management commands
+argument-hint: [command]
+---
+
+Manage epics (large features composed of multiple issues).
+
+## Available Commands
+
+- **status**: Show epic completion status
+  - Shows progress for each epic
+  - Lists child issues and their states
+  - Calculates completion percentage
+
+- **close-eligible**: Close epics where all children are complete
+  - Automatically closes epics when all child issues are done
+  - Useful for bulk epic cleanup
+
+## Epic Workflow
+
+1. Create epic: `bd create "Large Feature" -t epic -p 1`
+2. Link subtasks: `bd dep add bd-10 bd-20 --type parent-child` (epic bd-10 is parent of task bd-20)
+3. Track progress: `bd epic status`
+4. Auto-close when done: `bd epic close-eligible`
+
+Epics use parent-child dependencies to track subtasks.