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/create.md

@@ -0,0 +1,19 @@
+---
+description: Create a new issue interactively
+argument-hint: [title] [type] [priority]
+---
+
+Create a new beads issue. If arguments are provided:
+- $1: Issue title
+- $2: Issue type (bug, feature, task, epic, chore)
+- $3: Priority (0-4, where 0=critical, 4=backlog)
+
+If arguments are missing, ask the user for:
+1. Issue title (required)
+2. Issue type (default: task)
+3. Priority (default: 2)
+4. Description (optional)
+
+Use the beads MCP `create` tool to create the issue. Show the created issue ID and details to the user.
+
+Optionally ask if this issue should be linked to another issue (discovered-from, blocks, parent-child, related).