fix: defensive hardening — logging, dedup, locks, dead code

Four small fixes:

1. model_tools.py: Tool import failures logged at WARNING instead of
   DEBUG. If a tool module fails to import (syntax error, missing dep),
   the user now sees a warning instead of the tool silently vanishing.

2. hermes_cli/config.py: Remove duplicate 'import sys' (lines 19, 21).

3. agent/model_metadata.py: Remove 6 duplicate entries in
   DEFAULT_CONTEXT_LENGTHS dict. Python keeps the last value, so no
   functional change, but removes maintenance confusion.

4. hermes_state.py: Add missing self._lock to the LIKE query in
   resolve_session_id(). The exact-match path used get_session()
   (which locks internally), but the prefix fallback queried _conn
   without the lock.

model_tools.py

@@ -101,7 +101,7 @@ def _discover_tools():
         try:
             importlib.import_module(mod_name)
         except Exception as e:
-            logger.debug("Could not import %s: %s", mod_name, e)
+            logger.warning("Could not import tool module %s: %s", mod_name, e)
 
 
 _discover_tools()