feat(companion): Dross persona (Cradle) + migration 008 rename; remove dead API-key path

- system prompt = Dross (Ozriel's construct fragment, per Void 1.0), with tool guidance
- migration 008 renames the seeded agent 'companion' → display name 'Dross'
- removed lib/ai/anthropic.js + lib/ai/agent/runtime.js + tests + @anthropic-ai/sdk dep (companion now runs via the claude CLI; kept lib/ai/secret.js for the Vaultwarden roadmap)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

package.json

@@ -10,7 +10,6 @@
     "test:watch": "vitest"
   },
   "dependencies": {
-    "@anthropic-ai/sdk": "^0.40.1",
     "@modelcontextprotocol/sdk": "^1.29.0",
     "@mozilla/readability": "^0.6.0",
     "bcrypt": "^6.0.0",