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

[rka-oai]

What

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

[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.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Persist resolved rollout-budget defaults in config locks

These rollout-budget defaults are computed only during config load, but save_config_resolved_fields only materializes generic feature enablement and multi_agent_v2; it never writes config.rollout_budget back into the lock. When config-lock export/replay is used with a budget table that omits reminder_interval_tokens or either weight, the lock records only the partial TOML, so a later default change can replay the same session with different budget behavior while validation still passes. Serialize the resolved RolloutBudgetConfig into features.rollout_budget the same way multi_agent_v2 is handled.

AGENTS.md reference: AGENTS.md:L103-L110

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[rka-oai]

followed existing precedent from the multi agent feature. handled in 38c5b92