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/dump.py

@@ -32,11 +32,6 @@ def _get_git_commit(project_root: Path) -> str:
     return "(unknown)"
 
 
-def _key_present(name: str) -> str:
-    """Return 'set' or 'not set' for an env var."""
-    return "set" if os.getenv(name) else "not set"
-
-
 def _redact(value: str) -> str:
     """Redact all but first 4 and last 4 chars."""
     if not value: