feat(mcp): issue-capture convention in _INSTRUCTIONS (B8)

Finish the breakfix/issue-logging gap as a lightweight convention: when
recording a solved problem, capture symptom -> root cause -> fix and tag it
'issue' so it's findable instead of re-diagnosed. Pairs with the B9 trigger
('log when a problem is found'). No schema change — a structured note_type/
task_kind=issue is deferred to a joint schema pass with B7.

Refs plan 812 (B8 convention; B7 deferred).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/scribe/mcp/server.py

@@ -77,6 +77,9 @@ Keep task state honest — this is what makes the project a trustworthy record:
   that changes direction — write a short dated note on the project (create_note)
   capturing what happened (the pivots, not just the wins), and set the finished
   task to done.
+- When you record a problem you solved, capture symptom → root cause → fix
+  (tag it `issue`) so it's findable later — even one solved in passing is worth
+  two lines, so it isn't diagnosed from scratch next time.
 
 Scribe maintains a Rulebook system (Rulebook -> Topic -> Rule). Rules carry
 an actionable statement plus optional Why and How-to-apply context. At the