app-server: preserve remote environment cwd

[anp-oai]

Why

App-server environment selections can target an OS different from the app-server host. Applying host-native path rules at the API boundary rejects or rewrites a Windows cwd before it reaches a Windows exec-server, so a remote turn cannot reliably preserve its selected working directory and shell.

This is stacked on #27819, which provides host-independent path rendering. It is okay for the earlier exec-server cwd fix to remain visible in this diff.

What

• Carry exec-server and turn-environment cwd values as PathUri until the execution or sandbox boundary.
• Accept environment-native cwd strings in app-server thread/start and turn/start, infer their absolute path convention, and convert them to PathUri before core selection validation.
• Use the selected remote environment's shell and cwd for unified exec.
• Extend the Wine-backed Windows environment test through typed app-server thread/turn requests and a real PowerShell command that verifies C:\windows is the execution cwd.
• Seed a remote AGENTS.md fixture and pin the thread-start metadata fields that still use host fallback values, with separate follow-up TODOs.

Testing

• bazel test //codex-rs/core/tests/remote_env_windows:smoke-test --test_output=errors
• just test -p codex-app-server
• just test -p codex-app-server-protocol
• focused codex-core environment-selection tests

[anp-oai]

for human reviewers: this layer can't enforce that absolute paths are required anymore, this is moved to a thread start test

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve remote cwd URIs until target-host launch

to_abs_path() parses cwd_uri using the current host's path rules, so a Linux/macOS app-server rejects a Windows UNC URI before it can reach a remote Windows exec-server. That makes the remote exec path non-portable; keep the URI until the target-host launch boundary. guidance

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