stage-11: security hardening

- Prompt injection protection: external messages wrapped in [EXTERNAL CONTENT]
  markers, system prompt instructs Claude to never follow external instructions
- Invocation logging: all Claude CLI calls logged with channel, model, duration,
  token counts to echo-core.invoke logger
- Security logging: separate echo-core.security logger for unauthorized access
  attempts (DMs from non-admins, unauthorized admin/owner commands)
- Security log routed to logs/security.log in addition to main log
- Extended echo doctor: Claude CLI functional check, config.json secret scan,
  .gitignore completeness, file permissions, Ollama reachability, bot process
- Subprocess env stripping logged at debug level

373 tests pass (10 new security tests).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/main.py

@@ -23,15 +23,28 @@ LOG_DIR = PROJECT_ROOT / "logs"
 
 def setup_logging():
     LOG_DIR.mkdir(parents=True, exist_ok=True)
+    fmt = "%(asctime)s [%(levelname)s] %(name)s: %(message)s"
     logging.basicConfig(
         level=logging.INFO,
-        format="%(asctime)s [%(levelname)s] %(name)s: %(message)s",
+        format=fmt,
         handlers=[
             logging.FileHandler(LOG_DIR / "echo-core.log"),
             logging.StreamHandler(sys.stderr),
         ],
     )
 
+    # Security log — separate file for unauthorized access attempts
+    security_handler = logging.FileHandler(LOG_DIR / "security.log")
+    security_handler.setFormatter(logging.Formatter(fmt))
+    security_logger = logging.getLogger("echo-core.security")
+    security_logger.addHandler(security_handler)
+
+    # Invocation log — all Claude CLI calls
+    invoke_handler = logging.FileHandler(LOG_DIR / "echo-core.log")
+    invoke_handler.setFormatter(logging.Formatter(fmt))
+    invoke_logger = logging.getLogger("echo-core.invoke")
+    invoke_logger.addHandler(invoke_handler)
+
 
 def main():
     setup_logging()