canonicalize display of Agents.md paths on Windows. (#4577)

Canonicalize path on Windows to 
- remove unattractive path prefixes such as `\\?\`
- simplify it (`../AGENTS.md` vs
`C:\Users\iceweasel\code\coded\Agents.md`)
before: <img width="1110" height="45" alt="Screenshot 2025-10-01 123520"
src="https://github.com/user-attachments/assets/48920ae6-d89c-41b8-b4ea-df5c18fb5fad"
/>

after: 
<img width="585" height="46" alt="Screenshot 2025-10-01 123612"
src="https://github.com/user-attachments/assets/70a1761a-9d97-4836-b14c-670b6f13e608"
/>

codex-rs/core/src/project_doc.rs

@@ -14,6 +14,7 @@
 //! 3.  We do **not** walk past the Git root.
 
 use crate::config::Config;
+use dunce::canonicalize as normalize_path;
 use std::path::PathBuf;
 use tokio::io::AsyncReadExt;
 use tracing::error;
@@ -109,7 +110,7 @@ pub async fn read_project_docs(config: &Config) -> std::io::Result<Option<String
 /// is zero, returns an empty list.
 pub fn discover_project_doc_paths(config: &Config) -> std::io::Result<Vec<PathBuf>> {
     let mut dir = config.cwd.clone();
-    if let Ok(canon) = dir.canonicalize() {
+    if let Ok(canon) = normalize_path(&dir) {
         dir = canon;
     }