fix: remove 115 verified dead code symbols across 46 production files

Automated dead code audit using vulture + coverage.py + ast-grep intersection,
confirmed by Opus deep verification pass. Every symbol verified to have zero
production callers (test imports excluded from reachability analysis).

Removes ~1,534 lines of dead production code across 46 files and ~1,382 lines
of stale test code. 3 entire files deleted (agent/builtin_memory_provider.py,
hermes_cli/checklist.py, tests/hermes_cli/test_setup_model_selection.py).

Co-authored-by: alt-glitch <balyan.sid@gmail.com>

hermes_cli/commands.py

@@ -174,12 +174,6 @@ def resolve_command(name: str) -> CommandDef | None:
     return _COMMAND_LOOKUP.get(name.lower().lstrip("/"))
 
 
-def register_plugin_command(cmd: CommandDef) -> None:
-    """Append a plugin-defined command to the registry and refresh lookups."""
-    COMMAND_REGISTRY.append(cmd)
-    rebuild_lookups()
-
-
 def rebuild_lookups() -> None:
     """Rebuild all derived lookup dicts from the current COMMAND_REGISTRY.