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.lock

@@ -863,6 +863,7 @@ dependencies = [
  "codex-rmcp-client",
  "core_test_support",
  "dirs",
+ "dunce",
  "env-flags",
  "escargot",
  "eventsource-stream",
@@ -1200,6 +1201,7 @@ dependencies = [
  "crossterm",
  "diffy",
  "dirs",
+ "dunce",
  "image",
  "insta",
  "itertools 0.14.0",
@@ -1798,6 +1800,12 @@ version = "0.0.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f678cf4a922c215c63e0de95eb1ff08a958a81d47e485cf9da1e27bf6305cfa5"
 
+[[package]]
+name = "dunce"
+version = "1.0.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "92773504d58c093f6de2459af4af33faa518c13451eb8f2b5698ed3d36e7c813"
+
 [[package]]
 name = "dupe"
 version = "0.9.1"
@@ -4064,7 +4072,7 @@ dependencies = [
  "once_cell",
  "socket2 0.5.10",
  "tracing",
- "windows-sys 0.52.0",
+ "windows-sys 0.59.0",
 ]
 
 [[package]]