code-mode standalone: Add client and consume with packaging

[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
4. Remove v8 from core and only use IPC provider

Land and distribute the complete IPC path before making it the default.

• Add codex-code-mode-client to launch the host and manage IPC sessions.
• Make the core session provider injectable and add an opt-in IPC consumer path.
• Package the host alongside Codex in release archives and local builds.
• Add Bazel launchers and update installation scripts to locate and start the packaged host.
• Exercise core integration tests through the standalone host.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Do not fall back to an unbuilt code-mode host in Cargo tests

In a clean just test -p codex-core/nextest run, cargo_bin("codex-code-mode-host") has no CARGO_BIN_EXE_* for this other workspace package unless that binary was built separately; this fallback then uses the default provider, which only tries CODEX_CODE_MODE_HOST_PATH, a sibling of the test binary, or PATH. As a result, code-mode suite tests that construct TestCodex fail later with a spawn error instead of using a deterministic host path.

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[chatgpt-codex-connector]

Preserve missing-cell results after resume

If the first code-mode action after resuming an old rollout is wait for a cell ID that was yielded before shutdown, no live session has been recreated, so this path now returns code mode session is unavailable before asking the session for a MissingCell result. Previously an empty in-process session produced the normal exec cell … not found tool output, which lets resumed conversations recover from stale cell IDs instead of surfacing a code-mode infrastructure error.

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