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/schema/json/v2/ThreadStartResponse.json

@@ -2368,9 +2368,9 @@
     },
     "instructionSources": {
       "default": [],
-      "description": "Instruction source files currently loaded for this thread.",
+      "description": "Environment-native paths to instruction source files currently loaded for this thread.",
       "items": {
-        "$ref": "#/definitions/AbsolutePathBuf"
+        "$ref": "#/definitions/LegacyAppPathString"
       },
       "type": "array"
     },