[codex] simplify token budget context (#29295)

## Why

The token-budget feature currently adds remaining-token messages
whenever usage crosses the 25%, 50%, and 75% thresholds. Those periodic
inserts create prompt churn without requiring action, while the
near-compaction reminder and explicit `get_context_remaining` tool
already cover actionable and on-demand budget information.

The context-window lineage block is also easier to scan as plain labeled
text than as a `<token_budget>`-wrapped fragment.

## What changed

- Stop recording automatic remaining-token messages at percentage
thresholds.
- Render context-window lineage in `First`, `Current`, `Previous` order
with colon-separated labels.
- Omit the `Previous` line for the first context window.
- Remove `<token_budget>` wrappers from newly rendered lineage,
near-compaction reminders, and `get_context_remaining` output.
- Keep recognizing legacy wrapped fragments so existing rollouts remain
compatible.
- Remove the post-sampling token snapshot that was only needed by the
periodic threshold path.

## Testing

- `just test -p codex-core token_budget` (11 tests passed)

codex-rs/core/src/event_mapping.rs

@@ -34,6 +34,7 @@ const CONTEXTUAL_DEVELOPER_PREFIXES: &[&str] = &[
     REALTIME_CONVERSATION_OPEN_TAG,
     SKILLS_INSTRUCTIONS_OPEN_TAG,
     "<personality_spec>",
+    // Keep recognizing token-budget wrappers persisted by older versions.
     "<token_budget>",
     "<rollout_budget>",
 ];