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/update.md

@@ -0,0 +1,24 @@
+---
+description: Update an issue's status, priority, or other fields
+argument-hint: [issue-id] [status]
+---
+
+Update a beads issue.
+
+If arguments are provided:
+- $1: Issue ID
+- $2: New status (open, in_progress, blocked, closed)
+
+If arguments are missing, ask the user for:
+1. Issue ID
+2. What to update (status, priority, assignee, title, description)
+3. New value
+
+Use the beads MCP `update` tool to apply the changes. Show the updated issue to confirm the change.
+
+**Note:** Comments are managed separately with `bd comments add`. The `update` command is for singular, versioned properties (title, status, priority, etc.), while comments form a discussion thread that's appended to, not updated.
+
+Common workflows:
+- Start work: Update status to `in_progress`
+- Mark blocked: Update status to `blocked`
+- Reprioritize: Update priority (0-4)