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/model/mod.rs

@@ -3,6 +3,7 @@ mod backfill_state;
 mod graph;
 mod log;
 mod memories;
+mod thread_goal;
 mod thread_metadata;
 
 pub use agent_job::AgentJob;
@@ -25,6 +26,8 @@ pub use memories::Stage1JobClaimOutcome;
 pub use memories::Stage1Output;
 pub use memories::Stage1OutputRef;
 pub use memories::Stage1StartupClaimParams;
+pub use thread_goal::ThreadGoal;
+pub use thread_goal::ThreadGoalStatus;
 pub use thread_metadata::Anchor;
 pub use thread_metadata::BackfillStats;
 pub use thread_metadata::ExtractionOutcome;
@@ -38,6 +41,7 @@ pub(crate) use agent_job::AgentJobItemRow;
 pub(crate) use agent_job::AgentJobRow;
 pub(crate) use memories::Stage1OutputRow;
 pub(crate) use memories::stage1_output_ref_from_parts;
+pub(crate) use thread_goal::ThreadGoalRow;
 pub(crate) use thread_metadata::ThreadRow;
 pub(crate) use thread_metadata::anchor_from_item;
 pub(crate) use thread_metadata::datetime_to_epoch_millis;