feat: trigger memories from user turns with cooldown (#19970)

## Why

Memory startup was tied to thread lifecycle events such as create, load,
and fork. That can run memory work before a thread receives real user
input, and it makes startup cost scale with thread management instead of
actual turns. Moving the trigger to `thread/sendInput` keeps memory
startup aligned with the first real user turn and lets it use the
current thread config at turn time.

The idea is to prevent ghost cost due to pre-warm triggered by the app

Turn-based startup can also make global phase-2 consolidation easier to
request repeatedly, so this adds a success cooldown and tightens the
default startup scan window.

## What Changed

- Start `codex_memories_write::start_memories_startup_task` after a
non-empty `thread/sendInput` turn is submitted, instead of from thread
create/load/fork paths:
https://github.com/openai/codex/blob/d4a6885b7829e2fd2ec7a09355e4f75ebe1d1fe3/codex-rs/app-server/src/codex_message_processor.rs#L6477-L6487
- Expose `CodexThread::config()` so app-server can pass the live config
into memory startup at turn time.
- Add a six-hour successful-run cooldown for global phase-2
consolidation via `SkippedCooldown`:
https://github.com/openai/codex/blob/d4a6885b7829e2fd2ec7a09355e4f75ebe1d1fe3/codex-rs/state/src/runtime/memories.rs#L963-L966
- Reduce memory startup defaults to at most 2 rollouts over 10 days:
https://github.com/openai/codex/blob/d4a6885b7829e2fd2ec7a09355e4f75ebe1d1fe3/codex-rs/config/src/types.rs#L31-L34

## Verification

Updated the memory runtime coverage around phase-2 reclaim behavior,
including `phase2_global_lock_respects_success_cooldown`.

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/codex_thread.rs

@@ -34,6 +34,7 @@ use codex_utils_absolute_path::AbsolutePathBuf;
 use rmcp::model::ReadResourceRequestParams;
 use std::collections::HashMap;
 use std::path::PathBuf;
+use std::sync::Arc;
 use tokio::sync::Mutex;
 use tokio::sync::watch;
 
@@ -376,6 +377,10 @@ impl CodexThread {
         self.codex.thread_config_snapshot().await
     }
 
+    pub async fn config(&self) -> Arc<crate::config::Config> {
+        self.codex.session.get_config().await
+    }
+
     pub async fn read_mcp_resource(
         &self,
         server: &str,