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/core/src/custom_prompts.rs

@@ -0,0 +1,127 @@
+use codex_protocol::custom_prompts::CustomPrompt;
+use std::collections::HashSet;
+use std::path::Path;
+use std::path::PathBuf;
+use tokio::fs;
+
+/// Return the default prompts directory: `$CODEX_HOME/prompts`.
+/// If `CODEX_HOME` cannot be resolved, returns `None`.
+pub fn default_prompts_dir() -> Option<PathBuf> {
+    crate::config::find_codex_home()
+        .ok()
+        .map(|home| home.join("prompts"))
+}
+
+/// Discover prompt files in the given directory, returning entries sorted by name.
+/// Non-files are ignored. If the directory does not exist or cannot be read, returns empty.
+pub async fn discover_prompts_in(dir: &Path) -> Vec<CustomPrompt> {
+    discover_prompts_in_excluding(dir, &HashSet::new()).await
+}
+
+/// Discover prompt files in the given directory, excluding any with names in `exclude`.
+/// Returns entries sorted by name. Non-files are ignored. Missing/unreadable dir yields empty.
+pub async fn discover_prompts_in_excluding(
+    dir: &Path,
+    exclude: &HashSet<String>,
+) -> Vec<CustomPrompt> {
+    let mut out: Vec<CustomPrompt> = Vec::new();
+    let mut entries = match fs::read_dir(dir).await {
+        Ok(entries) => entries,
+        Err(_) => return out,
+    };
+
+    while let Ok(Some(entry)) = entries.next_entry().await {
+        let path = entry.path();
+        let is_file = entry
+            .file_type()
+            .await
+            .map(|ft| ft.is_file())
+            .unwrap_or(false);
+        if !is_file {
+            continue;
+        }
+        // Only include Markdown files with a .md extension.
+        let is_md = path
+            .extension()
+            .and_then(|s| s.to_str())
+            .map(|ext| ext.eq_ignore_ascii_case("md"))
+            .unwrap_or(false);
+        if !is_md {
+            continue;
+        }
+        let Some(name) = path
+            .file_stem()
+            .and_then(|s| s.to_str())
+            .map(|s| s.to_string())
+        else {
+            continue;
+        };
+        if exclude.contains(&name) {
+            continue;
+        }
+        let content = match fs::read_to_string(&path).await {
+            Ok(s) => s,
+            Err(_) => continue,
+        };
+        out.push(CustomPrompt {
+            name,
+            path,
+            content,
+        });
+    }
+    out.sort_by(|a, b| a.name.cmp(&b.name));
+    out
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::fs;
+    use tempfile::tempdir;
+
+    #[tokio::test]
+    async fn empty_when_dir_missing() {
+        let tmp = tempdir().expect("create TempDir");
+        let missing = tmp.path().join("nope");
+        let found = discover_prompts_in(&missing).await;
+        assert!(found.is_empty());
+    }
+
+    #[tokio::test]
+    async fn discovers_and_sorts_files() {
+        let tmp = tempdir().expect("create TempDir");
+        let dir = tmp.path();
+        fs::write(dir.join("b.md"), b"b").unwrap();
+        fs::write(dir.join("a.md"), b"a").unwrap();
+        fs::create_dir(dir.join("subdir")).unwrap();
+        let found = discover_prompts_in(dir).await;
+        let names: Vec<String> = found.into_iter().map(|e| e.name).collect();
+        assert_eq!(names, vec!["a", "b"]);
+    }
+
+    #[tokio::test]
+    async fn excludes_builtins() {
+        let tmp = tempdir().expect("create TempDir");
+        let dir = tmp.path();
+        fs::write(dir.join("init.md"), b"ignored").unwrap();
+        fs::write(dir.join("foo.md"), b"ok").unwrap();
+        let mut exclude = HashSet::new();
+        exclude.insert("init".to_string());
+        let found = discover_prompts_in_excluding(dir, &exclude).await;
+        let names: Vec<String> = found.into_iter().map(|e| e.name).collect();
+        assert_eq!(names, vec!["foo"]);
+    }
+
+    #[tokio::test]
+    async fn skips_non_utf8_files() {
+        let tmp = tempdir().expect("create TempDir");
+        let dir = tmp.path();
+        // Valid UTF-8 file
+        fs::write(dir.join("good.md"), b"hello").unwrap();
+        // Invalid UTF-8 content in .md file (e.g., lone 0xFF byte)
+        fs::write(dir.join("bad.md"), vec![0xFF, 0xFE, b'\n']).unwrap();
+        let found = discover_prompts_in(dir).await;
+        let names: Vec<String> = found.into_iter().map(|e| e.name).collect();
+        assert_eq!(names, vec!["good"]);
+    }
+}