fix(s03,s04,s20): remove dead safe_path, sync READMEs

- Delete safe_path entirely (following PR #483 approach) — no dead code
- Update all 3 READMEs per module to reflect read_file in Gate 2
- s04: also update permission_hook snippet in READMEs
- s20: remove safe_path (already replaced with inline resolution)

Closes #482

s04_hooks/README.en.md

@@ -108,7 +108,7 @@ def permission_hook(block):
         for pattern in DENY_LIST:
             if pattern in block.input.get("command", ""):
                 return "Permission denied by deny list"
-    if block.name in ("write_file", "edit_file"):
+    if block.name in ("read_file", "write_file", "edit_file"):
         path = block.input.get("path", "")
         if not (WORKDIR / path).resolve().is_relative_to(WORKDIR):
             choice = input("   Allow? [y/N] ").strip().lower()