Add goal persistence foundation (1 / 5) (#18073)

Adds the persisted goal foundation for the rest of the stack. This PR is
intentionally limited to feature flag and state-layer behavior;
app-server APIs, model tools, runtime continuation, and TUI UX are
layered in later PRs.

## Why

Goal mode needs durable thread-level state before clients or model tools
can safely build on it. The state layer needs to know whether a goal
exists, what objective it tracks, whether it is active, paused,
budget-limited, or complete, and how much time/token usage has already
been accounted.

## What changed

- Added the `goals` feature flag and generated config schema entry.
- Added the `thread_goals` state table and Rust model for persisted
thread goals.
- Added state runtime APIs for creating, replacing, updating, deleting,
and accounting goal usage.
- Added `goal_id`-based stale update protection so an old goal update
cannot overwrite a replacement.
- Kept this PR scoped to persistence and state runtime behavior, with no
app-server, model-facing, continuation, or TUI behavior yet.

## Verification

- Added state runtime coverage for goal creation, replacement, stale
update protection, status transitions, token-budget behavior, and usage
accounting.

codex-rs/state/src/runtime.rs

@@ -20,6 +20,7 @@ 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;
@@ -58,6 +59,7 @@ mod backfill;
 mod device_key;
 #[cfg(test)]
 mod device_key_tests;
+mod goals;
 mod logs;
 mod memories;
 mod remote_control;
@@ -66,6 +68,9 @@ mod test_support;
 mod threads;
 
 pub use device_key::DeviceKeyBindingRecord;
+pub use goals::ThreadGoalAccountingMode;
+pub use goals::ThreadGoalAccountingOutcome;
+pub use goals::ThreadGoalUpdate;
 pub use remote_control::RemoteControlEnrollmentRecord;
 pub use threads::ThreadFilterOptions;