Add exec-server transport for unified exec

[starr-openai]

Summary

Part 1

• add [exec_server].command config plumbing so experimental_exec_server can launch through an explicit command vector such as ssh -T dev ...
• resolve that launch command in codex-core, document it in docs/config.md, and cover the config parsing/launch-command behavior in tests
• propagate exec_server.command through codex exec thread start/resume so the exec-server override survives the app-server boundary

Part 2

• make remote exec-server startup more tolerant of host differences by remapping missing local shell paths to a usable remote shell family
• remap macOS /private/... cwd aliases to the stripped path when that directory exists remotely, which fixes /private/tmp vs /tmp over SSH
• preserve the original remote output receiver so very short one-off commands do not lose early stdout before unified-exec subscribes
• add focused coverage for the short-command remote-output path in codex-core

Testing

• cargo test -p codex-exec-server
• cargo test -p codex-core unified_exec_short_command_captures_output_through_exec_server -- --nocapture
• manual e2e:
  • build local codex
  • build remote codex-exec-server on dev
  • run:

cd /tmp
unset CODEX_EXEC_SERVER_EXE
/Users/starr/code/codex/codex-rs/target/debug/codex exec \
  --json \
  --skip-git-repo-check \
  --dangerously-bypass-approvals-and-sandbox \
  --enable experimental_exec_server \
  -c 'exec_server.command=["ssh","-o","ClearAllForwardings=yes","-T","dev","/home/dev-user/codex-rs/target/debug/codex-exec-server"]' \
  'Use exec_command to run `hostname` exactly once and then answer with only the stdout.'

• observed output: dev-0

Notes

• I did not run the full cargo test workspace suite after the core changes.

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