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/features/src/lib.rs

@@ -185,6 +185,8 @@ pub enum Feature {
     DefaultModeRequestUserInput,
     /// Enable automatic review for approval prompts.
     GuardianApproval,
+    /// Enable persisted thread goals and automatic goal continuation.
+    Goals,
     /// Enable collaboration modes (Plan, Default).
     /// Kept for config backward compatibility; behavior is always collaboration-modes-enabled.
     CollaborationModes,
@@ -928,6 +930,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
+    FeatureSpec {
+        id: Feature::Goals,
+        key: "goals",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::CollaborationModes,
         key: "collaboration_modes",