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/migrations/0029_thread_goals.sql

@@ -0,0 +1,11 @@
+CREATE TABLE thread_goals (
+    thread_id TEXT PRIMARY KEY NOT NULL REFERENCES threads(id) ON DELETE CASCADE,
+    goal_id TEXT NOT NULL,
+    objective TEXT NOT NULL,
+    status TEXT NOT NULL CHECK(status IN ('active', 'paused', 'budget_limited', 'complete')),
+    token_budget INTEGER,
+    tokens_used INTEGER NOT NULL DEFAULT 0,
+    time_used_seconds INTEGER NOT NULL DEFAULT 0,
+    created_at_ms INTEGER NOT NULL,
+    updated_at_ms INTEGER NOT NULL
+);