Custom /prompts (#2696)

Adds custom `/prompts` to `~/.codex/prompts/<command>.md`.

<img width="239" height="107" alt="Screenshot 2025-08-25 at 6 22 42 PM"
src="https://github.com/user-attachments/assets/fe6ebbaa-1bf6-49d3-95f9-fdc53b752679"
/>

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Details:

1. Adds `Op::ListCustomPrompts` to core.
2. Returns `ListCustomPromptsResponse` with list of `CustomPrompt`
(name, content).
3. TUI calls the operation on load, and populates the custom prompts
(excluding prompts that collide with builtins).
4. Selecting the custom prompt automatically sends the prompt to the
agent.

docs/prompts.md

@@ -0,0 +1,15 @@
+## Custom Prompts
+
+Save frequently used prompts as Markdown files and reuse them quickly from the slash menu.
+
+- Location: Put files in `$CODEX_HOME/prompts/` (defaults to `~/.codex/prompts/`).
+- File type: Only Markdown files with the `.md` extension are recognized.
+- Name: The filename without the `.md` extension becomes the slash entry. For a file named `my-prompt.md`, type `/my-prompt`.
+- Content: The file contents are sent as your message when you select the item in the slash popup and press Enter.
+- How to use:
+  - Start a new session (Codex loads custom prompts on session start).
+  - In the composer, type `/` to open the slash popup and begin typing your prompt name.
+  - Use Up/Down to select it. Press Enter to submit its contents, or Tab to autocomplete the name.
+- Notes:
+  - Files with names that collide with built‑in commands (e.g. `/init`) are ignored and won’t appear.
+  - New or changed files are discovered on session start. If you add a new prompt while Codex is running, start a new session to pick it up.