[codex] exec-server honors remote environment cwd and shell

[anp-oai]

Why

Next slice needed to make progress on the remote_env_windows test is to support passing a Windows cwd for the remote environment and using that environment's native shell. This lets the test run a real Windows process instead of only recording an early path or shell mismatch.

What

• change TurnEnvironmentSelection.cwd from AbsolutePathBuf to PathUri
• convert local cwd values to URIs when constructing selections
• preserve a remote primary cwd instead of replacing it with the local legacy fallback
• prefer the selected environment's discovered shell for unified exec, falling back to the session shell when unavailable
• convert back to a host-native absolute path at current native-only consumer boundaries
• reject or deny unsupported foreign cwd values at the existing request-permissions boundary, with TODOs for its future migration
• extend the hermetic Wine test to execute Windows PowerShell in C:\windows and verify successful process completion
• record the current app-server rejection against the same Wine-backed remote Windows fixture when its cwd is supplied as a native Windows path

[chatgpt-codex-connector]

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[pakrym-oai]

Will we use shell snapshot for local because of this/

[pakrym-oai]

That is (sadly) done in a different place.

[anp-oai]

Looks like unified_exec only wraps local environment execs in snapshots. Seems like we might need to figure out whether we want shell snapshots for remote environments but that's a later problem I think