Delete the eight functions nobody called

Measured, not guessed: every top-level def in every tracked .py file was
counted against every other tracked file, including markdown, scripts, Go
and TypeScript. Eight names had zero references anywhere:
cartridges_holding_atom and clear_mounts (memory.py reads each cartridge's
own sources.json, and set_mounts already clears by replacing the set),
gradient_checkpointing_enabled, group_hits_by_source, ingested_paths,
is_finite_number, parse_runtime_json (the runtime answers in prose now, and
the JSON path was never wired back), write_sources_json.

Typer commands and pytest cases were excluded from the count because a
framework calls them by name. The same scan now returns zero.

263 tests pass; repo-wide mypy is still 24 errors in 6 files, unchanged.

backend/src/ely_eye/runtime.py

@@ -1,7 +1,6 @@
 from __future__ import annotations
 
 import base64
-import json
 import logging
 import subprocess
 from dataclasses import dataclass
@@ -429,17 +428,3 @@ def gpu_status() -> dict[str, Any]:
     }
 
 
-def parse_runtime_json(text: str) -> dict[str, Any]:
-    stripped = text.strip()
-    if stripped.startswith("```"):
-        stripped = stripped.strip("`")
-        if stripped.lower().startswith("json"):
-            stripped = stripped[4:].strip()
-    try:
-        return json.loads(stripped)
-    except json.JSONDecodeError:
-        start = stripped.find("{")
-        end = stripped.rfind("}")
-        if start >= 0 and end > start:
-            return json.loads(stripped[start : end + 1])
-        raise