code-mode standalone: Cutover to new process

[cconger]

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This is phase 3 of a 4 phase stack:

1. Add protocol and host crates for new IPC code mode implementation code-mode standalone: extract protocol and add host crate #27724
2. Create the new standalone binary code-mode standalone: add host binary  #27725
3. Create a new IPC CodeModeSessionProvider to use new binary code-mode standalone: Add client and consume with packaging #27726
4. Remove v8 from core and only use IPC provider

Complete the process isolation and remove V8 from the core dependency graph.

• Make the IPC client the default code-mode session provider.
• Remove the direct codex-code-mode dependency from codex-core.
• Move remaining shared type and helper consumers onto codex-code-mode-protocol.
• Ensure V8 is linked only into the standalone host binary.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep code mode working in single-binary installs

When codex-code-mode-host is not installed beside the CLI (for example Cargo installs or single-binary release archives), this falls back to PATH and spawning fails, so code-mode execution regresses from the prior in-process behavior for those CLI install surfaces. guidance

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