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/Cargo.toml

@@ -27,6 +27,7 @@ codex-protocol = { workspace = true }
 codex-app-server-protocol = { workspace = true }
 codex-otel = { workspace = true, features = ["otel"] }
 dirs = { workspace = true }
+dunce = { workspace = true }
 env-flags = { workspace = true }
 eventsource-stream = { workspace = true }
 futures = { workspace = true }