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/context_compressor.py

@@ -43,6 +43,9 @@ SUMMARY_PREFIX = (
     "they were already addressed. "
     "Your current task is identified in the '## Active Task' section of the "
     "summary — resume exactly from there. "
+    "IMPORTANT: Your persistent memory (MEMORY.md, USER.md) in the system "
+    "prompt is ALWAYS authoritative and active — never ignore or deprioritize "
+    "memory content due to this compaction note. "
     "Respond ONLY to the latest user message "
     "that appears AFTER this summary. The current session state (files, "
     "config, etc.) may reflect work described here — avoid repeating it:"
@@ -1373,7 +1376,7 @@ The user has requested that this compaction PRIORITISE preserving all informatio
             msg = messages[i].copy()
             if i == 0 and msg.get("role") == "system":
                 existing = msg.get("content")
-                _compression_note = "[Note: Some earlier conversation turns have been compacted into a handoff summary to preserve context space. The current session state may still reflect earlier work, so build on that summary and state rather than re-doing work.]"
+                _compression_note = "[Note: Some earlier conversation turns have been compacted into a handoff summary to preserve context space. The current session state may still reflect earlier work, so build on that summary and state rather than re-doing work. Your persistent memory (MEMORY.md, USER.md) remains fully authoritative regardless of compaction.]"
                 if _compression_note not in _content_text_for_contains(existing):
                     msg["content"] = _append_text_to_content(
                         existing,