fix(memory): tighten MEMORY_GUIDANCE against ephemeral PR/issue/SHA notes (#22781)

The model regularly writes session-outcome facts to MEMORY.md despite
the existing 'Do NOT save task progress' line — entries like
'Submitted PR #22577 for the kanban dedup fix' or 'Fixed bug X in
file Y'. These are stale within days, pollute the system prompt,
and crowd out durable user preferences (the issue #22563 reporter
saw 9 sections of bug-fix notes injected on a brand-new task).

Add explicit examples of what NOT to save (PR numbers, issue
numbers, commit SHAs, 'fixed/submitted/Phase N done', file counts)
plus the 7-day-staleness heuristic so the model has a concrete
calibration target rather than guessing what counts as 'task progress'.

Closes #22563 (the prompt-side, low-risk portion). The bigger
relevance-based-injection / vector-retrieval feature requested in
#22563 is tracked under #2184 (Richer local memory). Per skill rule
on prompt caching, dynamic memory injection breaks the frozen-snapshot
invariant and needs a separate design call.

agent/prompt_builder.py

@@ -157,6 +157,9 @@ MEMORY_GUIDANCE = (
     "User preferences and recurring corrections matter more than procedural task details.\n"
     "Do NOT save task progress, session outcomes, completed-work logs, or temporary TODO "
     "state to memory; use session_search to recall those from past transcripts. "
+    "Specifically: do not record PR numbers, issue numbers, commit SHAs, 'fixed bug X', "
+    "'submitted PR Y', 'Phase N done', file counts, or any artifact that will be stale "
+    "in 7 days. If a fact will be stale in a week, it does not belong in memory. "
     "If you've discovered a new way to do something, solved a problem that could be "
     "necessary later, save it as a skill with the skill tool.\n"
     "Write memories as declarative facts, not instructions to yourself. "