refactor(memory): remove flush_memories entirely (#15696)

The AIAgent.flush_memories pre-compression save, the gateway
_flush_memories_for_session, and everything feeding them are
obsolete now that the background memory/skill review handles
persistent memory extraction.

Problems with flush_memories:

- Pre-dates the background review loop.  It was the only memory-save
  path when introduced; the background review now fires every 10 user
  turns on CLI and gateway alike, which is far more frequent than
  compression or session reset ever triggered flush.
- Blocking and synchronous.  Pre-compression flush ran on the live agent
  before compression, blocking the user-visible response.
- Cache-breaking.  Flush built a temporary conversation prefix
  (system prompt + memory-only tool list) that diverged from the live
  conversation's cached prefix, invalidating prompt caching.  The
  gateway variant spawned a fresh AIAgent with its own clean prompt
  for each finalized session — still cache-breaking, just in a
  different process.
- Redundant.  Background review runs in the live conversation's
  session context, gets the same content, writes to the same memory
  store, and doesn't break the cache.  Everything flush_memories
  claimed to preserve is already covered.

What this removes:

- AIAgent.flush_memories() method (~248 LOC in run_agent.py)
- Pre-compression flush call in _compress_context
- flush_memories call sites in cli.py (/new + exit)
- GatewayRunner._flush_memories_for_session + _async_flush_memories
  (and the 3 call sites: session expiry watcher, /new, /resume)
- 'flush_memories' entry from DEFAULT_CONFIG auxiliary tasks,
  hermes tools UI task list, auxiliary_client docstrings
- _memory_flush_min_turns config + init
- #15631's headroom-deduction math in
  _check_compression_model_feasibility (headroom was only needed
  because flush dragged the full main-agent system prompt along;
  the compression summariser sends a single user-role prompt so
  new_threshold = aux_context is safe again)
- The dedicated test files and assertions that exercised
  flush-specific paths

What this renames (with read-time backcompat on sessions.json):

- SessionEntry.memory_flushed -> SessionEntry.expiry_finalized.
  The session-expiry watcher still uses the flag to avoid re-running
  finalize/eviction on the same expired session; the new name
  reflects what it now actually gates.  from_dict() reads
  'expiry_finalized' first, falls back to the legacy 'memory_flushed'
  key so existing sessions.json files upgrade seamlessly.

Supersedes #15631 and #15638.

Tested: 383 targeted tests pass across run_agent/, agent/, cli/,
and gateway/ session-boundary suites.  No behavior regressions —
background memory review continues to handle persistent memory
extraction on both CLI and gateway.

tests/gateway/test_resume_command.py

@@ -4,7 +4,7 @@ Tests the _handle_resume_command handler (switch to a previously-named session)
 across gateway messenger platforms.
 """
 
-from unittest.mock import MagicMock, AsyncMock
+from unittest.mock import MagicMock
 
 import pytest
 
@@ -53,9 +53,6 @@ def _make_runner(session_db=None, current_session_id="current_session_001",
     mock_store.switch_session.return_value = mock_session_entry
     runner.session_store = mock_store
 
-    # Stub out memory flushing
-    runner._async_flush_memories = AsyncMock()
-
     return runner
 
 
@@ -233,28 +230,3 @@ class TestHandleResumeCommand:
 
         assert real_key not in runner._running_agents
         db.close()
-
-    @pytest.mark.asyncio
-    async def test_resume_flushes_memories(self, tmp_path):
-        """Resume should flush memories from the current session before switching."""
-        from hermes_state import SessionDB
-
-        db = SessionDB(db_path=tmp_path / "state.db")
-        db.create_session("old_session", "telegram")
-        db.set_session_title("old_session", "Old Work")
-        db.create_session("current_session_001", "telegram")
-
-        event = _make_event(text="/resume Old Work")
-        runner = _make_runner(
-            session_db=db,
-            current_session_id="current_session_001",
-            event=event,
-        )
-
-        await runner._handle_resume_command(event)
-
-        runner._async_flush_memories.assert_called_once_with(
-            "current_session_001",
-            "agent:main:telegram:dm:67890",
-        )
-        db.close()