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/Cargo.toml

@@ -97,6 +97,7 @@ derive_more = "2"
 diffy = "0.4.2"
 dirs = "6"
 dotenvy = "0.15.7"
+dunce = "1.0.4"
 env-flags = "0.1.1"
 env_logger = "0.11.5"
 escargot = "0.5"