feat(security): apply audited PR #6738 remediations and test hardening

- Enforce vision probe SSRF safe local host validation, SafeProbeBackend TOCTOU elimination, and session owner authorization
- Authorize session titling endpoints against registered owner configuration
- Sanitize sensitive metadata logging from agent loop debug logs
- Add async non-blocking execution for spec router tools and thinking decisions
- Harden email cache invalidation fencing and thread-safe mutation
- Add regression test suites for legacy browser authority, spec contracts, spec router request scoping, and task MCP delivery

tests/test_context_compactor.py

@@ -499,3 +499,20 @@ class TestResearchPrimerPreserved:
         trimmed = trim_for_context(msgs, context_length=1024, reserve_tokens=256)
         joined = "\n".join(str(m.get("content", "")) for m in trimmed)
         assert "You are Odysseus." in joined
+
+
+def test_unterminated_image_label_with_long_whitespace_finishes_promptly():
+    """Untrusted captions must not make image-history compaction quadratic."""
+    import subprocess
+    import sys
+
+    script = '''
+from src.context_compactor import keep_recent_image_turns
+messages = [
+    {'role': 'user', 'content': [{'type': 'text', 'text': '[Image:' + ' ' * 100000},
+                                 {'type': 'image_url', 'image_url': {'url': 'data:image/png;base64,AA=='}}]},
+    {'role': 'user', 'content': [{'type': 'image_url', 'image_url': {'url': 'data:image/png;base64,AA=='}}]},
+]
+assert '[Image]' in keep_recent_image_turns(messages, keep=1)[0]['content'][0]['text']
+'''
+    subprocess.run([sys.executable, '-c', script], check=True, timeout=8)