Remove remaining custom prompt support (#16115)

## Summary
- remove protocol and core support for discovering and listing custom
prompts
- simplify the TUI slash-command flow and command popup to built-in
commands only
- delete obsolete custom prompt tests, helpers, and docs references
- clean up downstream event handling for the removed protocol events

docs/prompts.md

@@ -1,3 +0,0 @@
-# Custom prompts
-
-For an overview of custom prompts, see [this documentation](https://developers.openai.com/codex/custom-prompts).

docs/tui-chat-composer.md

@@ -81,8 +81,6 @@ Key effects when disabled:
 
 - When `popups_enabled` is `false`, `sync_popups()` forces `ActivePopup::None`.
 - When `slash_commands_enabled` is `false`, the composer does not treat `/...` input as commands.
-- When `slash_commands_enabled` is `false`, the composer does not expand custom prompts in
-  `prepare_submission_text`.
 - When `slash_commands_enabled` is `false`, slash-context paste-burst exceptions are disabled.
 - When `image_paste_enabled` is `false`, file-path paste image attachment is skipped.
 - `ChatWidget` may toggle `image_paste_enabled` at runtime based on the selected model's
@@ -107,12 +105,8 @@ the input starts with `!` (shell command).
 
 1. Expands any pending paste placeholders so element ranges align with the final text.
 2. Trims whitespace and rebases element ranges to the trimmed buffer.
-3. Expands `/prompts:` custom prompts:
-   - Named args use key=value parsing.
-   - Numeric args use positional parsing for `$1..$9` and `$ARGUMENTS`.
-     The expansion preserves text elements and yields the final submission payload.
-4. Prunes attachments so only placeholders that survive expansion are sent.
-5. Clears pending pastes on success and suppresses submission if the final text is empty and there
+3. Prunes attachments so only placeholders that survive trimming are sent.
+4. Clears pending pastes on success and suppresses submission if the final text is empty and there
    are no attachments.
 
 The same preparation path is reused for slash commands with arguments (for example `/plan` and
@@ -124,16 +118,6 @@ still available for `Ctrl+Y`. This supports flows where a user kills part of a d
 composer action such as changing reasoning level, and then yanks that text back into the cleared
 draft.
 
-### Numeric auto-submit path
-
-When the slash popup is open and the first line matches a numeric-only custom prompt with
-positional args, Enter auto-submits without calling `prepare_submission_text`. That path still:
-
-- Expands pending pastes before parsing positional args.
-- Uses expanded text elements for prompt expansion.
-- Prunes attachments based on expanded placeholders.
-- Clears pending pastes after a successful auto-submit.
-
 ## Remote image rows (selection/deletion flow)
 
 Remote image URLs are shown as `[Image #N]` rows above the textarea, inside the same composer box.