[codex] add waypoints: multi-host remote execution

[nlieb-oai]

Summary

This adds a shared multi-host execution registry above individual Codex sessions. It preserves the existing local/default environment behavior while allowing registered hosts to be selected per exec_command invocation via host_id.

The initial host backend added here is SSH-backed. Codex starts a remote codex exec-server through the SSH host from ~/.ssh/config and proxies the websocket over the SSH command stream, which avoids requiring SSH TCP forwarding. The implementation also captures the remote default cwd and shell so unified exec can format shell commands for the selected host rather than blindly using the local shell.

Details

• Adds CODEX_EXEC_SERVER_SSH_HOSTS and CODEX_EXEC_SERVER_DEFAULT_HOST environment support.
• Adds a registry of local, URL-backed, and SSH-backed exec hosts.
• Adds optional host_id to the unified exec_command tool schema and approval key.
• Keeps the environment manager shared through sessions and sub-agents.
• Adds compatibility for older exec-server initialize responses that returned {} before sessionId existed.
• Adds an ignored SSH smoke test that can be run with any configured SSH host via CODEX_TEST_SSH_HOST.

Validation

• cargo fmt -p codex-exec-server -p codex-core -p codex-tools
• cargo check -p codex-exec-server -p codex-core -p codex-tools
• cargo check -p codex-tools
• cargo test -p codex-exec-server environment
• cargo test -p codex-exec-server initialize_response_accepts_legacy_empty_payload
• CODEX_TEST_SSH_HOST=<ssh-host> cargo test -p codex-exec-server environment::tests::ssh_environment_manager_bootstraps_configured_host -- --ignored --nocapture
• CODEX_SKIP_VENDORED_BWRAP=1 cargo test -p codex-core unified_exec:: --lib
• cargo test -p codex-tools
• git diff --check

[github-actions]

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[nlieb-oai]

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