[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_tests.rs

@@ -29,7 +29,7 @@ fn recognizes_skills_instructions_as_contextual_developer_content() {
 }
 
 #[test]
-fn recognizes_token_budget_as_contextual_developer_content() {
+fn recognizes_legacy_token_budget_as_contextual_developer_content() {
     let content = vec![ContentItem::InputText {
         text: "<token_budget>\nYou have 710 tokens left in this context window.\n</token_budget>"
             .to_string(),