Allow custom prompt or template for thread title generation

Feature request

Please add a supported way to customize the prompt or template Codex uses when generating thread titles.

Use case

I want generated titles to follow a consistent prefix pattern, for example:

• [✨ FEAT] feature work
• [⚠️ BUG] bugs, investigations, failing checks
• [♻️ REF] refactors or cleanup
• [🧭 PLAN] planning or research
• [📌 OTHER] admin/setup/simple questions

Example:

[⚠️ BUG] Checkout login redirect failure

Why

Thread history is easier to scan when generated titles follow a personal taxonomy. Manual rename and cleanup automations work, but they are after-the-fact. A title-generation prompt/template would make the first title correct.

Notes

This should affect only title generation, not the main agent prompt, and it should not overwrite titles the user manually set.

Related: #24289, #14482, #15444, #24202.