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

bazel/rules/testing/wine/src/lib.rs

@@ -105,6 +105,14 @@ impl WineTestCommand {
 }
 
 impl WineTestProcess {
+    /// Returns the host path to this process's isolated Wine prefix.
+    pub fn prefix_path(&self) -> &Path {
+        let Some(processes) = self.processes.as_ref() else {
+            panic!("Wine process guard is missing");
+        };
+        processes.prefix.path()
+    }
+
     /// Takes the piped standard output of the Wine process.
     ///
     /// This may only be called once for a process created by