Expose instruction sources (AGENTS.md) via app server (#17506)

Addresses #17498

Problem: The TUI derived /status instruction source paths from the local
client environment, which could show stale <none> output or incorrect
paths when connected to a remote app server.

Solution: Add an app-server v2 instructionSources snapshot to thread
start/resume/fork responses, default it to an empty list when older
servers omit it, and render TUI /status from that server-provided
session data.

Additional context: The app-server field is intentionally named
instructionSources rather than AGENTS.md-specific terminology because
the loaded instruction sources can include global instructions, project
AGENTS.md files, AGENTS.override.md, user-defined instruction files, and
future dynamic sources.

codex-rs/app-server-protocol/schema/json/v2/ThreadForkResponse.json

@@ -2187,6 +2187,14 @@
     "cwd": {
       "type": "string"
     },
+    "instructionSources": {
+      "default": [],
+      "description": "Instruction source files currently loaded for this thread.",
+      "items": {
+        "type": "string"
+      },
+      "type": "array"
+    },
     "model": {
       "type": "string"
     },