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.

gateway/session.py

@@ -439,11 +439,11 @@ class SessionEntry:
     auto_reset_reason: Optional[str] = None  # "idle" or "daily"
     reset_had_activity: bool = False  # whether the expired session had any messages
     
-    # Set by the background expiry watcher after it successfully flushes
-    # memories for this session.  Persisted to sessions.json so the flag
-    # survives gateway restarts (the old in-memory _pre_flushed_sessions
-    # set was lost on restart, causing redundant re-flushes).
-    memory_flushed: bool = False
+    # Set by the background expiry watcher after it finalizes an expired
+    # session (invoking on_session_finalize hooks and evicting the cached
+    # agent).  Persisted to sessions.json so the flag survives gateway
+    # restarts — prevents redundant finalization runs.
+    expiry_finalized: bool = False
 
     # When True the next call to get_or_create_session() will auto-reset
     # this session (create a new session_id) so the user starts fresh.
@@ -479,7 +479,7 @@ class SessionEntry:
             "last_prompt_tokens": self.last_prompt_tokens,
             "estimated_cost_usd": self.estimated_cost_usd,
             "cost_status": self.cost_status,
-            "memory_flushed": self.memory_flushed,
+            "expiry_finalized": self.expiry_finalized,
             "suspended": self.suspended,
             "resume_pending": self.resume_pending,
             "resume_reason": self.resume_reason,
@@ -531,7 +531,7 @@ class SessionEntry:
             last_prompt_tokens=data.get("last_prompt_tokens", 0),
             estimated_cost_usd=data.get("estimated_cost_usd", 0.0),
             cost_status=data.get("cost_status", "unknown"),
-            memory_flushed=data.get("memory_flushed", False),
+            expiry_finalized=data.get("expiry_finalized", data.get("memory_flushed", False)),
             suspended=data.get("suspended", False),
             resume_pending=data.get("resume_pending", False),
             resume_reason=data.get("resume_reason"),