fix: show ai once backend initialization status

[mikemikimike]

Summary

Closes #1408.

caro ai --once now writes an initialization status line to stderr before backend construction, so first-run model initialization is no longer silent. stdout remains reserved for the generated command text.

Implementation

• Added a stable initialization status message for the one-shot AI path.
• Emitted it immediately before CliApp::with_overrides(...).await.
• Added a focused regression test for the status message.

Testing

• cargo test --bin caro ai_once_initialization_message_is_stable — passed (1 passed, 22 filtered out).
• The command emitted two existing unreachable_code warnings in src/models/mod.rs and src/platform/mod.rs.
• git diff --check — passed.

Not run: full cargo test, cargo clippy -- -D warnings, and full formatting check. The full test command previously attempted to update crates.io and stalled in the network environment; offline mode lacked the cached rav1e crate. The repository-wide formatting check reports existing line-ending discrepancies across many files.

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Summary by cubic

Make caro ai --once report backend initialization so first runs aren’t silent. It now writes "Initializing backend…" to stderr right before backend construction; stdout remains command text only.

• Refactors

  • Removed a redundant AI initialization helper to use the same construction path.

• Migration

  • Update scripts that treat any stderr output as failure to allow this single status line.

^{Written for commit d9d417a. Summary will update on new commits.}

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_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}

[github-actions]

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[mikemikimike]

I have read the CLA Document and I hereby sign the CLA

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[cubic-dev-ai]

All reported issues were addressed across 1 file

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[mikemikimike]

I have read the CLA Document and I hereby sign the CLA

[cubic-dev-ai]

All reported issues were addressed across 1 file

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

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[mikemikimike]

recheck

[mikemikimike]

I rechecked this PR for issue #1408. The remaining red check is cla-check, and I have already signed the CLA in the PR comments. This is a repository contribution gate, not a code failure from the change. Could a maintainer please refresh or rerun the CLA check and review/approve the PR?