feat: add phase 1 mem db (#10634)

- Schema: thread_id (PK, FK to threads.id with cascade delete),
trace_summary, memory_summary, updated_at.
- Migration: creates the table and an index on (updated_at DESC,
thread_id DESC) for efficient recent-first reads.
  - Runtime API (DB-only):
      - `get_thread_memory(thread_id)`: fetch one memory row.
- `upsert_thread_memory(thread_id, trace_summary, memory_summary)`:
insert/update by thread id and always advance updated_at.
- `get_last_n_thread_memories_for_cwd(cwd, n)`: join thread_memory with
threads and return newest n rows for an exact cwd match.
- Model layer: introduced ThreadMemory and row conversion types to keep
query decoding typed and consistent with existing state models.

codex-rs/state/src/model/thread_memory.rs

@@ -0,0 +1,52 @@
+use anyhow::Result;
+use chrono::DateTime;
+use chrono::Utc;
+use codex_protocol::ThreadId;
+use sqlx::Row;
+use sqlx::sqlite::SqliteRow;
+
+/// Stored memory summaries for a single thread.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ThreadMemory {
+    pub thread_id: ThreadId,
+    pub trace_summary: String,
+    pub memory_summary: String,
+    pub updated_at: DateTime<Utc>,
+}
+
+#[derive(Debug)]
+pub(crate) struct ThreadMemoryRow {
+    thread_id: String,
+    trace_summary: String,
+    memory_summary: String,
+    updated_at: i64,
+}
+
+impl ThreadMemoryRow {
+    pub(crate) fn try_from_row(row: &SqliteRow) -> Result<Self> {
+        Ok(Self {
+            thread_id: row.try_get("thread_id")?,
+            trace_summary: row.try_get("trace_summary")?,
+            memory_summary: row.try_get("memory_summary")?,
+            updated_at: row.try_get("updated_at")?,
+        })
+    }
+}
+
+impl TryFrom<ThreadMemoryRow> for ThreadMemory {
+    type Error = anyhow::Error;
+
+    fn try_from(row: ThreadMemoryRow) -> std::result::Result<Self, Self::Error> {
+        Ok(Self {
+            thread_id: ThreadId::try_from(row.thread_id)?,
+            trace_summary: row.trace_summary,
+            memory_summary: row.memory_summary,
+            updated_at: epoch_seconds_to_datetime(row.updated_at)?,
+        })
+    }
+}
+
+fn epoch_seconds_to_datetime(secs: i64) -> Result<DateTime<Utc>> {
+    DateTime::<Utc>::from_timestamp(secs, 0)
+        .ok_or_else(|| anyhow::anyhow!("invalid unix timestamp: {secs}"))
+}