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/exec/src/lib_tests.rs

@@ -410,6 +410,7 @@ fn session_configured_from_thread_response_uses_review_policy_from_response() {
         model_provider: "openai".to_string(),
         service_tier: None,
         cwd: PathBuf::from("/tmp"),
+        instruction_sources: Vec::new(),
         approval_policy: codex_app_server_protocol::AskForApproval::OnRequest,
         approvals_reviewer: codex_app_server_protocol::ApprovalsReviewer::GuardianSubagent,
         sandbox: codex_app_server_protocol::SandboxPolicy::WorkspaceWrite {