Feature request: configurable prompt snippets in the TUI composer

What variant of Codex are you using?

CLI

What feature would you like to see?

Summary

I would like to add configurable prompt snippets to the Codex CLI TUI composer.

The intended workflow is:

1. The user types a normal task prompt.
2. When the cursor is after a trigger such as --, Codex shows a small picker with user-defined phrases/snippets.
3. The user selects one item with keyboard navigation.
4. Codex inserts the selected phrase at the cursor position immediately.

## Motivation

Some users repeatedly add the same task constraints, verification requirements, coding preferences, or workflow
instructions to prompts. A lightweight snippet picker would reduce repetitive typing while keeping the final prompt
visible and editable before submission.

Examples of snippets:

• "追根溯源，从根源处修复，避免臃肿实现。"
• "改动后运行相关测试，并说明验证结果。"
• "保持改动最小，不要重构无关代码。"

## Proposed behavior

• Snippets are user-configurable.
• The trigger could be configurable, with a default such as -- or space + --.
• The picker should work inside the existing TUI prompt composer.
• Selecting a snippet inserts plain text into the current prompt.
• It should not send the prompt automatically.
• It should not affect slash commands such as /model, /review, /skills, etc.

## Possible configuration shape

For example:

```toml
[prompt_snippets]
trigger = "--"

[[prompt_snippets.items]]
label = "Root-cause fix"
text = "Trace the issue to its root cause and keep the fix minimal."

[[prompt_snippets.items]]
label = "Run tests"
text = "Run the relevant tests and report the verification result."

## Question for maintainers

Would this be an acceptable feature direction for Codex CLI?

If yes, I would like to implement it and submit a PR, following the contribution process.

Additional information

_No response_