feat(plugin): auto-inject retrieves on the conversation, not the typed words alone (#4364)

The prompt hook sent only the operator's message, so a mid-session
follow-up ("yes do that") named nothing a note, lesson or rule could
match. The hook now reads the tail of the last assistant reply from the
transcript and sends it as `ctx`; the notes and rule arms append it to a
short prompt (<= 280 chars), prompt first, capped at 600 chars. No model
tokens: it is embedding input, and the injected menu's budget is unchanged.

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

src/scribe/routes/plugin.py

@@ -95,6 +95,12 @@ async def autoinject_retrieve():
         project_id (opt)   — explicit project scope override (ad-hoc/testing).
         exclude_ids (opt)  — comma-separated note ids already injected this
                              session; skipped so each note injects at most once.
+        ctx (opt)          — the tail of the last assistant reply (#4364). The
+                             notes AND rule arms append it to a short prompt,
+                             so a follow-up like "yes do that" still names
+                             what it is about — and a rule or lesson arrives
+                             while the work is under way, before the operator
+                             has to call it out.
         exclude_rule_ids   — comma-separated rule ids already surfaced this
             (opt)            session. SHARED with /prior-art and /tool-rules on
                              purpose: one session keeps ONE rule ledger, so a
@@ -125,11 +131,13 @@ async def autoinject_retrieve():
     exclude_rule_ids = _int_list(request.args.get("exclude_rule_ids"))
     held_rule_ids = _int_list(request.args.get("held_rule_ids"))
 
+    ctx = request.args.get("ctx") or ""
     rules = await plugin_ctx_svc.build_prompt_rule_hint(
-        g.user.id, q, project_id=project_id, exclude_rule_ids=exclude_rule_ids, held_rule_ids=held_rule_ids
+        g.user.id, q, project_id=project_id, exclude_rule_ids=exclude_rule_ids, held_rule_ids=held_rule_ids,
+        context=ctx,
     )
     result = await plugin_ctx_svc.build_autoinject_hint(
-        g.user.id, q, project_id=project_id, exclude_ids=exclude_ids
+        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]
     result["context"] = "\n\n".join(blocks)