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/label.md

@@ -0,0 +1,33 @@
+---
+description: Manage issue labels
+argument-hint: [command] [issue-id] [label]
+---
+
+Manage labels on beads issues. Labels provide flexible cross-cutting metadata beyond structured fields (status, priority, type).
+
+## Available Commands
+
+- **add**: Add a label to an issue
+  - $1: "add"
+  - $2: Issue ID
+  - $3: Label name
+
+- **remove**: Remove a label from an issue
+  - $1: "remove"
+  - $2: Issue ID
+  - $3: Label name
+
+- **list**: List labels on a specific issue
+  - $1: "list"
+  - $2: Issue ID
+
+- **list-all**: Show all labels used across all issues
+
+## Common Label Use Cases
+
+- Technical scope: `backend`, `frontend`, `api`, `database`
+- Quality gates: `needs-review`, `needs-tests`, `security-review`
+- Effort sizing: `quick-win`, `complex`, `spike`
+- Context: `technical-debt`, `documentation`, `performance`
+
+Use `bd label add <issue-id> <label>` to tag issues with contextual metadata.