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/app-server-protocol/src/protocol/common.rs

@@ -2573,7 +2573,11 @@ mod tests {
                 service_tier: None,
                 cwd,
                 runtime_workspace_roots: Vec::new(),
-                instruction_sources: vec![absolute_path("/tmp/AGENTS.md")],
+                instruction_sources: vec![
+                    codex_utils_path_uri::LegacyAppPathString::from_abs_path(&absolute_path(
+                        "/tmp/AGENTS.md",
+                    )),
+                ],
                 approval_policy: v2::AskForApproval::OnFailure,
                 approvals_reviewer: v2::ApprovalsReviewer::User,
                 sandbox: v2::SandboxPolicy::DangerFullAccess,