Avoid rewriting Phase 2 selection on clean workspace (#19812)

## Why

Phase 2 can now claim the global consolidation lock on startup even when
the git-backed memory workspace is already clean. The clean-workspace
path still finalized through the normal Phase 2 success path, which
clears and re-marks `selected_for_phase2` rows. That made no-op startups
perform avoidable writes to `stage1_outputs`, creating unnecessary DB
I/O and contention when no memory files changed.

## What Changed

- Added a preserving-selection Phase 2 finalizer in `codex-state` that
only marks the global job row as succeeded.
- Kept the existing `mark_global_phase2_job_succeeded` behavior for real
consolidation runs, where the selected Phase 2 snapshot must be
rewritten.
- Switched the `succeeded_no_workspace_changes` branch in
`core/src/memories/phase2.rs` to use the preserving-selection finalizer.
- Added a regression test that installs a SQLite trigger on
`stage1_outputs` and verifies the clean finalizer performs zero updates
there.

## Testing

- `cargo test -p codex-state`
- `cargo test -p codex-core memories::tests::phase2`

codex-rs/core/src/memories/phase2.rs

@@ -122,12 +122,11 @@ pub(super) async fn run(session: &Arc<Session>, config: Arc<Config>) {
     if !workspace_diff.has_changes() {
         tracing::error!("Phase 2 no changes");
         // We check only after sync of the file system.
-        job::succeed(
+        job::succeed_preserving_selection(
             session,
             db,
             &claim,
             new_watermark,
-            &raw_memories,
             "succeeded_no_workspace_changes",
         )
         .await;
@@ -291,6 +290,23 @@ mod job {
             .await
             .unwrap_or(false)
     }
+
+    pub(super) async fn succeed_preserving_selection(
+        session: &Arc<Session>,
+        db: &StateRuntime,
+        claim: &Claim,
+        completion_watermark: i64,
+        reason: &'static str,
+    ) -> bool {
+        session.services.session_telemetry.counter(
+            metrics::MEMORY_PHASE_TWO_JOBS,
+            /*inc*/ 1,
+            &[("status", reason)],
+        );
+        db.mark_global_phase2_job_succeeded_preserving_selection(&claim.token, completion_watermark)
+            .await
+            .unwrap_or(false)
+    }
 }
 
 mod agent {