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/test_services_plugin_context.py

@@ -1,7 +1,8 @@
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
-from tests.helpers import fake_note
+from scribe.services import retrieval_surfaces as rs
+from tests.helpers import fake_note, writepath_cfg
 
 
 pytestmark = pytest.mark.usefixtures("_no_supersession")
@@ -17,7 +18,8 @@ async def test_get_autoinject_config_defaults_and_clamps():
     from scribe.services import plugin_context as pc
 
     # No settings stored → defaults.
-    with patch.object(pc, "get_setting", AsyncMock(side_effect=lambda uid, k, d: d)):
+    with patch.object(pc, "get_setting", AsyncMock(side_effect=lambda uid, k, d: d)), \
+         patch.object(rs, "get_setting", AsyncMock(side_effect=lambda uid, k, d="": d)):
         cfg = await pc.get_autoinject_config(1)
     assert cfg == {
         "enabled": pc.AUTOINJECT_DEFAULT_ENABLED,
@@ -31,8 +33,12 @@ async def test_get_autoinject_config_defaults_and_clamps():
         pc.AUTOINJECT_THRESHOLD_KEY: "5",
         pc.AUTOINJECT_TOP_K_KEY: "999",
     }
-    with patch.object(pc, "get_setting",
-                      AsyncMock(side_effect=lambda uid, k, d: stored.get(k, d))):
+    # The switch comes from plugin_context, the two numbers from the registry.
+    def _side(uid, k, d=""):
+        return stored.get(k, d)
+
+    with patch.object(pc, "get_setting", AsyncMock(side_effect=_side)), \
+         patch.object(rs, "get_setting", AsyncMock(side_effect=_side)):
         cfg = await pc.get_autoinject_config(1)
     assert cfg["enabled"] is False
     assert cfg["threshold"] == 1.0
@@ -437,9 +443,7 @@ async def test_write_path_semantic_arm_asks_for_experience_not_just_snippets():
     search = AsyncMock(return_value=hits)
     rec = MagicMock()
     with patch.object(pc, "get_writepath_config",
-                      AsyncMock(return_value={"enabled": True, "threshold": 0.6,
-                                              "top_k": 3,
-                                              "rule_threshold": 0.72})), \
+                      AsyncMock(return_value=writepath_cfg(threshold=0.6))), \
          patch.object(pc.snippets_svc, "list_snippets",
                       AsyncMock(return_value=([], 0))), \
          patch.object(pc, "semantic_search_notes", search), \
@@ -470,9 +474,7 @@ async def test_write_path_labels_a_non_snippet_hit_with_its_kind():
     hits = [(0.72, fake_note(id=9, title="debounce helper", user_id=1, note_type="snippet")),
             (0.71, fake_note(id=7, title="Debounce dropped the trailing call", user_id=1, is_task=True, task_kind="issue"))]
     with patch.object(pc, "get_writepath_config",
-                      AsyncMock(return_value={"enabled": True, "threshold": 0.6,
-                                              "top_k": 3,
-                                              "rule_threshold": 0.72})), \
+                      AsyncMock(return_value=writepath_cfg(threshold=0.6))), \
          patch.object(pc.snippets_svc, "list_snippets",
                       AsyncMock(return_value=([], 0))), \
          patch.object(pc, "semantic_search_notes", AsyncMock(return_value=hits)), \