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/init.md

@@ -0,0 +1,18 @@
+---
+description: Initialize beads in the current project
+argument-hint: [prefix]
+---
+
+Initialize beads issue tracking in the current directory.
+
+If a prefix is provided as $1, use it as the issue prefix (e.g., "myproject" creates issues like myproject-1, myproject-2). If not provided, the default is the current directory name.
+
+Use the beads MCP `init` tool with the prefix parameter (if provided) to set up a new beads database.
+
+After initialization:
+1. Show the database location
+2. Show the issue prefix that will be used
+3. Explain the basic workflow (or suggest running `/beads:workflow`)
+4. Suggest creating the first issue with `/beads:create`
+
+If beads is already initialized, inform the user and show project stats using the `stats` tool.