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/lib.rs

@@ -209,13 +209,28 @@ impl PathUri {
         PathBuf::from(self.inferred_native_path_string())
     }
 
-    /// Returns the parent URI, or `None` for the URI root or an opaque fallback
-    /// URI created by [`Self::from_abs_path`].
+    /// Returns the lexical parent without crossing the inferred native path root.
+    ///
+    /// POSIX `/`, Windows drive roots, Windows UNC share roots, and opaque fallback
+    /// URIs created by [`Self::from_abs_path`] have no parent.
     pub fn parent(&self) -> Option<Self> {
-        if self.encoded_path() == "/" || decode_bad_path_uri(&self.0).is_some() {
+        if decode_bad_path_uri(&self.0).is_some() {
             return None;
         }
 
+        let convention = self.infer_path_convention()?;
+        // In URI form, both a Windows drive root (`file:///C:`) and a UNC share root
+        // (`file://server/share`) retain one non-empty path segment. Keep that segment as the
+        // anchor so parent traversal cannot produce a URI that is not an absolute Windows path.
+        let anchor_depth = usize::from(convention == PathConvention::Windows);
+        let depth = self
+            .0
+            .path_segments()?
+            .filter(|segment| !segment.is_empty())
+            .count();
+        if depth <= anchor_depth {
+            return None;
+        }
         let mut url = self.0.clone();
         {
             let mut segments = match url.path_segments_mut() {
@@ -227,6 +242,11 @@ impl PathUri {
         Some(Self(url))
     }
 
+    /// Returns this URI and each lexical parent up to its inferred native path root.
+    pub fn ancestors(&self) -> impl Iterator<Item = Self> {
+        std::iter::successors(Some(self.clone()), Self::parent)
+    }
+
     /// Lexically resolves native absolute or relative path text against this URI.
     ///
     /// Path text is interpreted using the POSIX or Windows convention inferred