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"
/>

---

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.

codex-rs/protocol/src/custom_prompts.rs

@@ -0,0 +1,10 @@
+use serde::Deserialize;
+use serde::Serialize;
+use std::path::PathBuf;
+
+#[derive(Serialize, Deserialize, Debug, Clone)]
+pub struct CustomPrompt {
+    pub name: String,
+    pub path: PathBuf,
+    pub content: String,
+}