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/delete.md

@@ -0,0 +1,32 @@
+---
+description: Delete issues and clean up references
+argument-hint: [issue-ids...] [--force]
+---
+
+Delete one or more issues and clean up all references.
+
+## Safety Features
+
+- **Preview mode**: Default shows what would be deleted
+- **--force**: Required to actually delete
+- **--dry-run**: Preview collision detection
+- **Dependency checks**: Fails if issue has dependents (unless --cascade or --force)
+
+## Batch Deletion
+
+- Delete multiple: `bd delete bd-1 bd-2 bd-3 --force`
+- Delete from file: `bd delete --from-file deletions.txt --force`
+
+## Dependency Handling
+
+- **Default**: Fails if issue has dependents not in deletion set
+- **--cascade**: Recursively delete all dependent issues
+- **--force**: Delete and orphan dependents
+
+## What Gets Deleted
+
+1. All dependency links (any type, both directions)
+2. Text references updated to "[deleted:ID]" in connected issues
+3. Issue removed from database
+
+This operation cannot be undone. Use with caution!