security: block secret exfiltration via browser URLs and auxiliary LLM calls

Three exfiltration vectors closed:

1. Browser URL exfil — agent could embed secrets in URL params and
   navigate to attacker-controlled server. Now scans URLs for known
   API key patterns before navigating (browser_navigate, web_extract).

2. Browser snapshot leak — page displaying env vars or API keys would
   send secrets to auxiliary LLM via _extract_relevant_content before
   run_agent.py's redaction layer sees the result. Now redacts snapshot
   text before the auxiliary call.

3. Camofox annotation leak — accessibility tree text sent to vision
   LLM could contain secrets visible on screen. Now redacts annotation
   context before the vision call.

10 new tests covering URL blocking, snapshot redaction, and annotation
redaction for both browser and camofox backends.

tools/browser_camofox.py

@@ -485,6 +485,12 @@ def camofox_vision(question: str, annotate: bool = False,
             except Exception:
                 pass
 
+        # Redact secrets from annotation context before sending to vision LLM.
+        # The screenshot image itself cannot be redacted, but at least the
+        # text-based accessibility tree snippet won't leak secret values.
+        from agent.redact import redact_sensitive_text
+        annotation_context = redact_sensitive_text(annotation_context)
+
         # Send to vision LLM
         from agent.auxiliary_client import call_llm