docs(retrieval): stop asking the operator to diagnose retrieval (#4102)

The last item in this step's done-when: no wording anywhere asks an
operator to tune for correctness.

Four Settings hints told them to do exactly that — "raise it if rules
keep arriving unread", "lower it if a git push arrives with nothing",
"lower this if genuine duplicates go unnoticed". Every one of those
asks the operator to diagnose a ranker from symptoms, which is the job
the model now does from the records: the telemetry says what each bar
refused, and reading those records is what separates a real miss from
a bar doing its job. The hints keep the explanation of WHAT each number
is — that is worth reading — and drop the homework.

`plugin_context.py` said the defaults "are meant to be tuned from
retrieval_logs once data accrues", which was true and had no owner.
It now names who does it and with what.

`retrieval_telemetry`'s docstring gained the warning that belongs
beside it: this readout has been measured pointing the wrong way, so
the ids it returns are the point, not its percentiles.

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

src/scribe/services/plugin_context.py

@@ -56,9 +56,14 @@ _GOAL_CHARS = 200
 # Per-user settings (keys live in the generic settings table). The threshold is
 # deliberately STRICTER than the pull-search default (embeddings
 # DEFAULT_SIMILARITY_THRESHOLD = 0.45): an unsolicited per-turn inject must clear
-# a higher bar than a search the agent chose to run. Defaults start conservative
-# and are meant to be tuned from retrieval_logs (source='auto_inject') once data
-# accrues — they're exposed in the Settings UI, no restart needed.
+# a higher bar than a search the agent chose to run.
+#
+# The defaults below are STARTING POINTS, and correcting them is the model's
+# job, not the operator's (#4102): `retrieval_surfaces` says what is in force,
+# `retrieval_telemetry(near_miss_samples=N)` says what it refused, and
+# `tune_retrieval` moves it with the reason attached. The operator can set any
+# of them in Settings and their change is recorded the same way — but nobody
+# has to read a log to get correct behaviour out of this.
 AUTOINJECT_ENABLED_KEY = "kb_autoinject_enabled"
 # The key and the value both live in the registry now (#4102); these names
 # survive because the comments above them are where each number's measurement