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/close.md

@@ -0,0 +1,18 @@
+---
+description: Close a completed issue
+argument-hint: [issue-id] [reason]
+---
+
+Close a beads issue that's been completed.
+
+If arguments are provided:
+- $1: Issue ID
+- $2+: Completion reason (optional)
+
+If the issue ID is missing, ask for it. Optionally ask for a reason describing what was done.
+
+Use the beads MCP `close` tool to close the issue. Show confirmation with the issue details.
+
+After closing, suggest checking for:
+- Dependent issues that might now be unblocked (use `ready` tool)
+- New work discovered during this task (use `create` tool with `discovered-from` link)