chore(deps): add @huggingface/transformers for prompt injection classifier

Dependency needed for the ML prompt injection defense layer coming in the
follow-up commits. @huggingface/transformers will host the TestSavantAI
BERT-small classifier that scans tool outputs for indirect prompt injection.

Note: this dep only runs in non-compiled bun contexts (sidebar-agent.ts).
The compiled browse binary cannot load it because transformers.js v4 requires
onnxruntime-node (native module, fails to dlopen from bun compile's temp
extract dir). See docs/designs/ML_PROMPT_INJECTION_KILLER.md for the full
architectural decision.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -38,6 +38,7 @@
     "slop:diff": "bun run scripts/slop-diff.ts"
   },
   "dependencies": {
+    "@huggingface/transformers": "^4.1.0",
     "@ngrok/ngrok": "^1.7.0",
     "diff": "^7.0.0",
     "playwright": "^1.58.2",