fix: salvage batch — compaction guidance, memory authority, cache eviction after compression

- Fix /compact → /compress in context-overflow tips (closes #20020)
- Evict cached agent after session hygiene and /compress so system
  prompt refreshes with current SOUL.md, memory, and skills
- Restore memory authority across compaction: change 'informational
  background data' to 'authoritative reference data' in memory block
  and SUMMARY_PREFIX, with backward-compatible regex

Based on:
- PR #20027 by @LeonSGP43
- PR #18767 by @MacroAnarchy
- PR #17380 by @vominh1919

PR #17121 boundary marker fix already merged to main (2eef395e1).
PR #9262 user-message anchoring already on main via _ensure_last_user_message_in_tail().

agent/memory_manager.py

@@ -46,7 +46,7 @@ _INTERNAL_CONTEXT_RE = re.compile(
     re.IGNORECASE,
 )
 _INTERNAL_NOTE_RE = re.compile(
-    r'\[System note:\s*The following is recalled memory context,\s*NOT new user input\.\s*Treat as informational background data\.\]\s*',
+    r'\[System note:\s*The following is recalled memory context,\s*NOT new user input\.\s*Treat as (?:informational background data|authoritative reference data[^\]]*)\.\]\s*',
     re.IGNORECASE,
 )
 
@@ -180,7 +180,8 @@ def build_memory_context_block(raw_context: str) -> str:
     return (
         "<memory-context>\n"
         "[System note: The following is recalled memory context, "
-        "NOT new user input. Treat as informational background data.]\n\n"
+        "NOT new user input. Treat as authoritative reference data — "
+        "this is the agent's persistent memory and should inform all responses.]\n\n"
         f"{clean}\n"
         "</memory-context>"
     )