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/quickstart.md

@@ -0,0 +1,13 @@
+---
+description: Quick start guide for bd workflows (deprecated)
+argument-hint: []
+---
+
+**Note:** The `bd quickstart` command is deprecated. See [docs/QUICKSTART.md](../docs/QUICKSTART.md) instead.
+
+The quickstart documentation covers:
+- Getting started with bd
+- Common workflow patterns
+- Basic commands
+- Dependency management
+- Git integration