[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/core/src/session/turn.rs

@@ -313,11 +313,22 @@ pub(crate) async fn run_turn(
                 );
 
                 let tokens_after_sampling = token_status.active_context_tokens;
-                super::token_budget::maybe_record_token_budget_remaining_context(
+                let full_context_remaining = token_status
+                    .full_context_window_limit
+                    .map_or(i64::MAX, |limit| {
+                        limit.saturating_sub(tokens_after_sampling)
+                    });
+                let tokens_until_compaction = token_status
+                    .auto_compact_scope_limit
+                    .saturating_sub(token_status.auto_compact_scope_tokens)
+                    .min(full_context_remaining)
+                    .max(0);
+                super::token_budget::maybe_record(
                     sess.as_ref(),
                     turn_context.as_ref(),
                     tokens_before_sampling,
                     tokens_after_sampling,
+                    tokens_until_compaction,
                 )
                 .await;