[codex] add rollout token budget configuration (varlength 1/N) (#28746)

## What

This PR defines the structured configuration contract for shared rollout
token budgets (across ALL agent threads under 1 rollout).

```toml
[features.rollout_budget]
enabled = true
limit_tokens = 100000
reminder_interval_tokens = 10000
sampling_token_weight = 1.0
prefill_token_weight = 0.1
```

The reminder interval defaults to 10% of the rollout limit. Sampling and
prefill weights default to `1.0`.

## Scope

This PR only defines and validates configuration. It does not track
usage, inject reminders, or stop a rollout. Accounting and reminders are
implemented in the stacked follow-up #28494.

The existing `token_budget` feature remains unchanged. `rollout_budget`
has its own feature key and configuration type.

## Tests

The config test verifies that the structured fields resolve into
`RolloutBudgetConfig` and do not enable the existing `token_budget`
feature.

Local checks:

- `just write-config-schema`
- `just test -p codex-core load_config_resolves_rollout_budget`
- `cargo check -p codex-thread-manager-sample`
- `git diff --check`

The full workspace test suite was not run locally.

codex-rs/thread-manager-sample/src/main.rs

@@ -277,6 +277,7 @@ fn new_config(model: Option<String>, arg0_paths: Arg0DispatchPaths) -> anyhow::R
         background_terminal_max_timeout: 300_000,
         ghost_snapshot: GhostSnapshotConfig::default(),
         multi_agent_v2: MultiAgentV2Config::default(),
+        rollout_budget: None,
         features: Default::default(),
         suppress_unstable_features_warning: false,
         active_project: ProjectConfig { trust_level: None },