[codex] simplify token budget context

[pakrym-oai]

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)