chore: isolate thread goal storage behind GoalStore (#23295)

## Why

Thread goal persistence is being prepared for a dedicated storage
boundary. Before that split, goal-specific reads, writes, accounting,
and cleanup were exposed directly on `StateRuntime`, so core and
app-server callsites stayed coupled to the full runtime instead of a
goal-specific store.

This PR introduces that boundary without changing the goal wire API or
current persistence behavior. Callers now go through
`StateRuntime::thread_goals()` and the new `GoalStore`, while
`GoalStore` still uses the existing state DB pool underneath.

## What changed

- Added `GoalStore` in `state/src/runtime/goals.rs` and exposed it from
`StateRuntime` via `thread_goals()`.
- Moved thread-goal reads, writes, status updates, pause, delete, and
usage accounting onto `GoalStore`.
- Updated core session goal handling, app-server goal RPCs, resume
snapshots, and goal tests to use the store boundary.
- Kept thread deletion responsible for cascading goal cleanup by
deleting the goal through the store only after a thread row is removed.

## Testing

- Existing goal persistence, resume, and accounting tests were updated
to exercise the new `GoalStore` access path.

codex-rs/state/src/runtime.rs

@@ -18,7 +18,6 @@ use crate::apply_rollout_item;
 use crate::migrations::runtime_logs_migrator;
 use crate::migrations::runtime_state_migrator;
 use crate::model::AgentJobRow;
-use crate::model::ThreadGoalRow;
 use crate::model::ThreadRow;
 use crate::model::anchor_from_item;
 use crate::model::datetime_to_epoch_millis;
@@ -65,6 +64,7 @@ mod remote_control;
 mod test_support;
 mod threads;
 
+pub use goals::GoalStore;
 pub use goals::ThreadGoalAccountingMode;
 pub use goals::ThreadGoalAccountingOutcome;
 pub use goals::ThreadGoalUpdate;
@@ -86,6 +86,7 @@ pub struct StateRuntime {
     default_provider: String,
     pool: Arc<sqlx::SqlitePool>,
     logs_pool: Arc<sqlx::SqlitePool>,
+    thread_goals: GoalStore,
     thread_updated_at_millis: Arc<AtomicI64>,
 }
 
@@ -164,6 +165,7 @@ impl StateRuntime {
         let thread_updated_at_millis = thread_updated_at_millis_result?;
         let thread_updated_at_millis = thread_updated_at_millis.unwrap_or(0);
         let runtime = Arc::new(Self {
+            thread_goals: GoalStore::new(Arc::clone(&pool)),
             pool,
             logs_pool,
             codex_home,
@@ -183,6 +185,10 @@ impl StateRuntime {
     pub fn codex_home(&self) -> &Path {
         self.codex_home.as_path()
     }
+
+    pub fn thread_goals(&self) -> &GoalStore {
+        &self.thread_goals
+    }
 }
 
 fn base_sqlite_options(path: &Path) -> SqliteConnectOptions {