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/show.md

@@ -0,0 +1,17 @@
+---
+description: Show detailed information about an issue
+argument-hint: [issue-id]
+---
+
+Display detailed information about a beads issue.
+
+If an issue ID is provided as $1, use it. Otherwise, ask the user for the issue ID.
+
+Use the beads MCP `show` tool to retrieve issue details and present them clearly, including:
+- Issue ID, title, and description
+- Status, priority, and type
+- Creation and update timestamps
+- Dependencies (what this issue blocks or is blocked by)
+- Related issues
+
+If the issue has dependencies, offer to show the full dependency tree.