refactor(retrieval): one registry for every surface's floor and budget (#4102)

Groundwork for the step's real change. The operator's decision is that the
floor is chosen and adjusted by the model using it, not shipped as a value
somebody has to defend:

  "we need a model consistent surface for the adjustment of these floor values.
   the user should be able to touch it but the model should be the thing
   handling it 9 times out of 10."

A tuning surface cannot be consistent across six arms that each spell their
configuration differently, so the arms stop owning their numbers.
`retrieval_surfaces.SURFACES` names each one, its floor key and default, its
budget key and default, and — because they are rendered by the tuning tool and
the Settings UI — what it asks, over what corpus, and how often it fires. A
floor cannot be moved responsibly by anyone who does not know those three.

Three things fall out:

- **`k` becomes a real budget everywhere.** Only auto-inject had a configurable
  one; `RULEHINT_LIMIT`, `PROMPTRULE_LIMIT` and `reply_preferences.LIMIT` were
  constants. `k` is what binds under a low floor, so it has to be settable per
  surface — and per surface is the point, since `pre_tool_rule` fires before
  every Bash call while `prompt_rule` fires once a turn.
- **`write_path` gets its own budget, inherited not reset.** It shared
  auto-inject's outright on the argument that "how many titles at once" means
  the same thing on both. It does not, for the same reason. Unset, it still
  reads auto-inject's key, so an install that tuned the shared knob does not
  silently drop to a new default.
- **The duplicated read-and-clamp goes.** That shape is canon #2860 across 295
  of 372 judged siblings. Survivable while the numbers were constants; not once
  they are meant to move.

The long measurement comments stay exactly where they are — #2223's noise-floor
probe, #3853's command-vs-code split, #3851's band measurement. The constants
they annotate now alias the registry, so there is one value and the reasoning
still sits beside it.

Tests build the write-path config from the registry (`helpers.writepath_cfg`)
instead of from hand-written dicts. That is not tidiness: the rule arms read
their numbers inside a fail-open `except`, so a dict missing one key does not
raise where a reader would see it — the arm silently becomes a no-op that reads
exactly like "fired and found nothing". Ten hand-written dicts each looked
complete on the day they were typed.

tests/test_retrieval_surfaces.py pins the identity everything rests on: a
surface's name IS its telemetry source. Nothing in the type system says so —
`record_retrieval(source="pre_tool_rule")` is a literal in another file — and
renaming one without the other yields an arm that can be tuned and not
measured, or measured and not tuned, with no symptom either way.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

tests/helpers.py

@@ -322,3 +322,32 @@ def http_sink(reply: bytes = b'{"context":"","note_ids":[]}'):
     finally:
         server.shutdown()
         server.server_close()
+
+
+def writepath_cfg(**over):
+    """A complete `get_writepath_config` stand-in, built from the registry (#4102).
+
+    DERIVED, NOT LITERAL, and the reason is a failure mode this file already
+    warned about in prose without being able to prevent: the write-path hint
+    drives three arms, each of which reads its numbers out of the config dict
+    inside a fail-open `except`. A dict missing one key does not raise where a
+    reader would see it — the arm silently becomes a no-op, which is
+    indistinguishable from the arm working and finding nothing.
+
+    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.
+    """
+    from scribe.services.retrieval_surfaces import SURFACES
+
+    cfg = {
+        "enabled": True,
+        "threshold": SURFACES["write_path"].floor_default,
+        "top_k": SURFACES["write_path"].budget_default,
+        "rule_threshold": SURFACES["write_path_rule"].floor_default,
+        "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,
+    }
+    cfg.update(over)
+    return cfg