docs: reflect normalization fixes, detector cleanup and CLI overrides

- README: remove stale detector_response.py tree entry and the
  detector-models highlight (module removed); document per-branch
  measured transition endpoints from paceENSDF and JSON-flow CLI
  override semantics
- TODO: drop the DEFAULT_JSON_SCHEMA detector-keys clause (removed)
- data/custom_input_example.json: strip dead detector_* keys

data/custom_input_example.json

@@ -5,14 +5,5 @@
     "A_number": 99,
     "endpoint_MeV": 0.294,
     "transition_type": "A",
-    "e_step_MeV": 0.001,
-    "use_detector_response": false,
-    "detector_model": "gaussian",
-    "detector_sigma_a_keV": 1.0,
-    "detector_sigma_b": 0.0,
-    "detector_tail_fraction": 0.0,
-    "detector_tau_keV": 5.0,
-    "detector_fano_factor": 0.12,
-    "detector_n_channels": 4096,
-    "detector_channel_energy_range": [0.0, 0.35]
+    "e_step_MeV": 0.001
 }