[codex] add configurable token budget compaction reminder (#29255)

## Why

The token-budget feature reports coarse remaining-context milestones,
but it does not give the model a configurable wrap-up prompt before
automatic compaction. A strict threshold-crossing check can also miss
resumed or reconfigured windows that are already inside the threshold.

## What changed

- Add structured `[features.token_budget]` configuration for an absolute
`reminder_threshold_tokens` and bounded `reminder_message_template`;
`{n_remaining}` is expanded when the reminder is delivered.
- Compute remaining tokens against the next effective auto-compaction
boundary, including scoped `body_after_prefix` accounting and the full
context-window limit.
- Make reminder delivery level-triggered before and after sampling, with
one-shot state owned by `AutoCompactWindow` and re-armed on compaction,
`new_context`, restore, or history replacement.
- Leave the existing initial full-window token-budget context, 25/50/75%
notices, and token-budget tools unchanged.
- Persist the resolved feature configuration in the session config lock
and regenerate the config schema.

## Validation

- `just test -p codex-core token_budget`
- `just test -p codex-core
token_budget_reminder_emits_after_crossing_compaction_threshold`
- `just test -p codex-core auto_compact_window`
- `just test -p codex-core
lock_contains_prompts_and_materializes_features`
- `just test -p codex-features`
- `just test -p codex-config`

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

@@ -280,6 +280,7 @@ 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(),
+        token_budget: None,
         rollout_budget: None,
         current_time_reminder: None,
         features: Default::default(),