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/rename-prefix.md

@@ -0,0 +1,24 @@
+---
+description: Rename the issue prefix for all issues
+argument-hint: <new-prefix> [--dry-run]
+---
+
+Rename the issue prefix for all issues in the database.
+
+Updates all issue IDs and all text references across all fields.
+
+## Prefix Rules
+
+- Max length: 8 characters
+- Allowed: lowercase letters, numbers, hyphens
+- Must start with a letter
+- Must end with a hyphen (e.g., 'kw-', 'work-')
+
+## Usage
+
+- **Preview**: `bd rename-prefix kw- --dry-run`
+- **Apply**: `bd rename-prefix kw-`
+
+Example: Rename from 'knowledge-work-' to 'kw-'
+
+All dependencies and text references are automatically updated.