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/src/thread_state.rs

@@ -13,7 +13,7 @@ use codex_protocol::ThreadId;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::RolloutItem;
 use codex_rollout::state_db::StateDbHandle;
-use codex_utils_absolute_path::AbsolutePathBuf;
+use codex_utils_path_uri::LegacyAppPathString;
 use std::collections::HashMap;
 use std::collections::HashSet;
 use std::sync::Arc;
@@ -31,7 +31,7 @@ pub(crate) struct PendingThreadResumeRequest {
     pub(crate) request_id: ConnectionRequestId,
     pub(crate) history_items: Vec<RolloutItem>,
     pub(crate) config_snapshot: ThreadConfigSnapshot,
-    pub(crate) instruction_sources: Vec<AbsolutePathBuf>,
+    pub(crate) instruction_sources: Vec<LegacyAppPathString>,
     pub(crate) thread_summary: codex_app_server_protocol::Thread,
     pub(crate) emit_thread_goal_update: bool,
     pub(crate) thread_goal_state_db: Option<StateDbHandle>,
@@ -200,6 +200,7 @@ mod tests {
     use codex_protocol::config_types::CollaborationMode;
     use codex_protocol::config_types::ModeKind;
     use codex_protocol::config_types::Settings;
+    use codex_utils_absolute_path::AbsolutePathBuf;
     use pretty_assertions::assert_eq;
 
     #[test]