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/.claude-plugin/plugin.json

@@ -0,0 +1,42 @@
+{
+  "name": "beads",
+  "description": "AI-supervised issue tracker for coding workflows. Manage tasks, discover work, and maintain context with simple CLI commands.",
+  "version": "0.46.0",
+  "author": {
+    "name": "Steve Yegge",
+    "url": "https://github.com/steveyegge"
+  },
+  "repository": "https://github.com/steveyegge/beads",
+  "license": "MIT",
+  "homepage": "https://github.com/steveyegge/beads",
+  "keywords": [
+    "issue-tracker",
+    "task-management",
+    "ai-workflow",
+    "agent-memory"
+  ],
+  "hooks": {
+    "SessionStart": [
+      {
+        "matcher": "",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "bd prime"
+          }
+        ]
+      }
+    ],
+    "PreCompact": [
+      {
+        "matcher": "",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "bd prime"
+          }
+        ]
+      }
+    ]
+  }
+}