fix(tests): the config stand-in fell behind the real one, and ten arms silently no-opped (#4214)

CI 7124 Python tests: 14 failed. One root cause behind ten of them, and the
failure was the exact one `tests/helpers.writepath_cfg`'s docstring already
warns about in prose — while being unable to prevent this instance of it.

Three arms read their numbers out of that config dict inside a fail-open
`except`. A missing key raises where nobody sees it, so the arm becomes a
silent no-op, indistinguishable from the arm working and finding nothing. The
helper derives its keys from `retrieval_surfaces.SURFACES` precisely to stop
that — and `checkpoint_threshold` is deliberately NOT a surface, because
everything in that table is a floor/budget pair belonging to one query and the
checkpoint runs none. The derivation therefore could not see it, the write-path
rule arm died before `record_retrieval`, and ten tests went red at once.

Fixed at the helper, from the module constant, so there is still exactly one
literal and it lives in the product. And the guard the docstring claimed now
exists: `test_the_config_stand_in_carries_every_key_the_real_one_does`
compares the stand-in's key set against the real `get_writepath_config`, so
the next key that is not a surface fails loudly here instead of quietly
disabling an arm under test. `test_retrieval_surfaces`'s hand-written key list
gains it for the same reason, spelled out in place.

The other four were the contract widening itself: `checkpoint` is present on
every return of the tool arm, including its early ones, so four assertions
comparing the whole dict needed it. That key is deliberately always present —
the two arms feed one shell reader where an absent key and an empty one are
read the same, so the difference is invisible exactly where it would bite.

Verified statically: every cfg in the suite now routes through `writepath_cfg`
(test_write_path_trigger's local `_cfg` delegates to it), no hand-written
config dict survives, and the real config's eight keys match the stand-in's.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

tests/helpers.py

@@ -374,7 +374,18 @@ def writepath_cfg(**over):
     So the keys come from `retrieval_surfaces.SURFACES`. A seventh surface, or a
     rename, changes this helper for free and cannot quietly disable an arm in
     ten hand-written dicts that each looked complete on the day they were typed.
+
+    NOT EVERY KEY IS A SURFACE, and that gap already bit once (#4214). The
+    checkpoint bar has no entry in SURFACES on purpose — everything in that
+    table is a floor/budget pair belonging to one QUERY, and the checkpoint
+    runs none, it re-reads hits the rule arms already produced. Adding it there
+    would give it a phantom budget. So it is taken from the module constant
+    instead, which keeps the "one literal, in the product" property even though
+    the derivation differs. `test_the_config_stand_in_carries_every_key_the_
+    real_one_does` is what makes the next addition fail loudly here rather than
+    silently no-op an arm, which is the whole claim this docstring makes.
     """
+    from scribe.services.plugin_context import _CHECKPOINT_DEFAULT
     from scribe.services.retrieval_surfaces import SURFACES
 
     cfg = {
@@ -385,6 +396,7 @@ def writepath_cfg(**over):
         "rule_top_k": SURFACES["write_path_rule"].budget_default,
         "tool_rule_threshold": SURFACES["pre_tool_rule"].floor_default,
         "tool_rule_top_k": SURFACES["pre_tool_rule"].budget_default,
+        "checkpoint_threshold": _CHECKPOINT_DEFAULT,
     }
     cfg.update(over)
     return cfg