Make AGENTS.md discovery FS-aware (#15826)

## Summary
- make AGENTS.md discovery and loading fully FS-aware and remove the
non-FS discover helper
- migrate remote-aware codex-core tests to use TestEnv workspace setup
instead of syncing a local workspace copy
- add AGENTS.md corner-case coverage, including directory fallbacks and
remote-aware integration coverage

## Testing
- cargo test -p codex-core project_doc -- --nocapture
- cargo test -p codex-core hierarchical_agents -- --nocapture
- cargo test -p codex-core agents_md -- --nocapture
- cargo test -p codex-tui status -- --nocapture
- cargo test -p codex-tui-app-server status -- --nocapture
- just fix
- just fmt
- just bazel-lock-update
- just bazel-lock-check
- just argument-comment-lint
- remote Linux executor tests in progress via scripts/test-remote-env.sh

codex-rs/core/src/codex_tests.rs

@@ -2612,14 +2612,16 @@ async fn session_new_fails_when_zsh_fork_enabled_without_zsh_path() {
         agent_status_tx,
         InitialHistory::New,
         SessionSource::Exec,
-        Arc::new(codex_exec_server::EnvironmentManager::new(
-            /*exec_server_url*/ None,
-        )),
         skills_manager,
         plugins_manager,
         mcp_manager,
         Arc::new(SkillsWatcher::noop()),
         AgentControl::default(),
+        Some(Arc::new(
+            codex_exec_server::Environment::create(/*exec_server_url*/ None)
+                .await
+                .expect("create environment"),
+        )),
     )
     .await;