Route unified exec process startup through remote exec-server

[starr-openai]

Summary

• add experimental config for a websocket exec-server endpoint plus optional remote workspace root mapping
• route unified exec process/session creation through a remote backend when configured
• wrap remote exec-server sessions behind UnifiedExecProcess so exec_command can stream output, write stdin, and terminate over the existing protocol
• sketch the filesystem end-state by making codex_environment::Environment pluggable and adding a remote exec-server filesystem adapter
• add a narrow smoke test for codex exec using a local websocket exec-server as the remote stand-in

Scope

• this draft now includes both the remote exec_command MVP and a filesystem-backend shape sketch so we can review the end state before splitting into smaller PRs
• no project-doc, skills, or app-server command routing changes in this draft
• remote mode fails fast for sandboxed exec and inherited FDs instead of falling back to local

Validation

• cargo check -p codex-core --lib
• cargo check -p codex-exec-server --lib
• cargo check -p codex-app-server --lib
• just fmt
• just fix -p codex-core -p codex-exec-server -p codex-app-server
• earlier on the first commit: cargo test -p codex-core --lib --no-run --message-format=short
• earlier on the first commit: cargo test -p codex-exec --test all remote_exec_server -- --nocapture

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[pakrym-oai]

This should be exec_server_url