core: load AGENTS.md from foreign environments (#28958)

## Why

Make it possible to load AGENTS.md from remote exec-servers whose OS is
different than app-server.

## What

- keep `AGENTS.md` discovery and provenance as `PathUri`, with
root-aware parent and ancestor traversal
- expose lifecycle instruction sources as legacy app-server path strings
in events while retaining `PathUri` internally
- preserve and test mixed POSIX and Windows paths in model context and
TUI status output
- cover remote Windows loading end to end by seeding the Wine prefix
through host filesystem APIs
- fix bug in `PathUri`'s parent() implementation that would erase
Windows drive letters

codex-rs/utils/path-uri/src/tests.rs

@@ -522,7 +522,7 @@ fn path_buf_uses_the_inferred_native_spelling() {
 }
 
 #[test]
-fn parent_uses_uri_hierarchy_and_preserves_authority() {
+fn parent_stops_at_posix_drive_and_unc_roots() {
     for (input, expected) in [
         (
             "file:///workspace/src/lib.rs",
@@ -531,12 +531,13 @@ fn parent_uses_uri_hierarchy_and_preserves_authority() {
         ("file:///workspace", Some("file:///")),
         ("file:///", None),
         ("file:///C:/Users", Some("file:///C:")),
-        ("file:///C:/", Some("file:///")),
+        ("file:///C:/", None),
+        ("file:///C:", None),
         (
             "file://server/share/src/main.rs",
             Some("file://server/share/src"),
         ),
-        ("file://server/share", Some("file://server/")),
+        ("file://server/share", None),
     ] {
         let uri = PathUri::parse(input).expect("valid file URI");
         let expected = expected.map(|value| PathUri::parse(value).expect("valid expected URI"));
@@ -544,6 +545,35 @@ fn parent_uses_uri_hierarchy_and_preserves_authority() {
     }
 }
 
+#[test]
+fn ancestors_include_self_and_stop_at_native_path_roots() {
+    for (input, expected) in [
+        (
+            "file:///workspace/src",
+            vec!["file:///workspace/src", "file:///workspace", "file:///"],
+        ),
+        (
+            "file:///C:/workspace/src",
+            vec![
+                "file:///C:/workspace/src",
+                "file:///C:/workspace",
+                "file:///C:",
+            ],
+        ),
+        (
+            "file://server/share/project",
+            vec!["file://server/share/project", "file://server/share"],
+        ),
+    ] {
+        let uri = PathUri::parse(input).expect("valid file URI");
+        let ancestors = uri
+            .ancestors()
+            .map(|path| path.to_string())
+            .collect::<Vec<_>>();
+        assert_eq!(ancestors, expected, "ancestors for {input}");
+    }
+}
+
 #[test]
 fn join_normalizes_relative_uri_segments() {
     for (base, relative, expected) in [