feat(lessons): soft links — a lesson and a rule arriving together in distinct situations are proposed as a link (milestone 440 step 3, #4637)

When one hook response puts a lesson and a rule in front of the reader, the
pair is recorded as evidence on a SUGGESTED lesson_rule_links row; once the
pair has arrived together in PROPOSE_SITUATIONS (3) distinct situations, the
next co-arrival carries one line asking the reader to judge it with
judge_lesson_link. Nothing about surfacing changes: a suggested link carries
no rule anywhere (that is #4633, confirmed links only).

- lesson_rules: co_surfaced (fail-open; only pairs the reader could confirm —
  a lesson they may write, a rule they own; judged pairs gather nothing; one
  proposal per response; PROPOSE_COOLDOWN 6h between asks), plus the pure
  counting rules: situation_key, add_evidence, proposal_due, evidence_summary.
  A situation is the prompt on /retrieve (word tokens, sorted and
  de-duplicated, so trivial rewordings count once) and the FILE on
  /prior-art (every edit to one file is one situation).
- rules_for_lessons shows a suggested link's evidence counts.
- plugin_context: build_autoinject_hint returns lesson_ids,
  build_prompt_rule_hint returns shown_rule_ids, build_write_path_hint records
  its own pair; routes/plugin /retrieve records the prompt pair. Shown lines,
  repeats included: relevance makes a co-arrival, not the session ledger.
- Evidence lives in the existing evidence column — no migration, and backup
  already carries it.
- Tests: counting rules (unit), the recorder against Postgres (bar, repeat,
  judged pairs, ownership, cooldown, evidence kept on confirm); conftest stubs
  co_surfaced for unit tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scribe/routes/plugin.py

@@ -12,6 +12,7 @@ from quart import Blueprint, g, jsonify, request
 
 from scribe.auth import admin_required, get_current_user_id, login_required
 from scribe.config import Config
+from scribe.services import lesson_rules as lesson_rules_svc
 from scribe.services import plugin_context as plugin_ctx_svc
 from scribe.services import repo_bindings as repo_bindings_svc
 from scribe.services import report_check as report_check_svc
@@ -147,7 +148,14 @@ async def autoinject_retrieve():
     result = await plugin_ctx_svc.build_autoinject_hint(
         g.user.id, q, project_id=project_id, exclude_ids=exclude_ids, context=ctx,
     )
-    blocks = [b for b in (rules["context"], result["context"]) if b]
+    # The one place both arms' lines meet, so the one place a lesson and a
+    # rule arriving TOGETHER can be counted (#4637). Keyed on the operator's
+    # prompt — the situation — and fails open to "".
+    proposal = await lesson_rules_svc.co_surfaced(
+        g.user.id, result.get("lesson_ids") or [], rules.get("shown_rule_ids") or [],
+        arm="p", situation=q, project_id=project_id or None,
+    )
+    blocks = [b for b in (rules["context"], result["context"], proposal) if b]
     result["context"] = "\n\n".join(blocks)
     result["rule_ids"] = rules["rule_ids"]
     return jsonify(result)