CommandFailure: a command's error prints as the command words it

[odrobnik]

In-process commands bridged through Shell.parsableCommand(_:) couldn't fail the way the tool they port does. SwiftPorts' git throws its CLIError with git's exact words and status. In an embedding shell (iBash on iOS), the bridge handed that to ArgumentParser's formatter, so this:

$ git fetch foo://example.invalid/x
Error: git: 'remote-foo' is not a git command. See 'git --help'.
fatal: remote helper 'foo' aborted session
[exit 1]

came out where git prints the two lines without Error: and exits 128. Every failing git command in iBash showed Error: error: … and exit 1.

Change

• New CommandFailure protocol in ShellKit: an Error with a diagnostic (written to stderr as is) and an exitStatus.
• ParsableCommandBridge catches it before falling back to ArgumentParser's formatter.

Commands that don't conform are unchanged. SwiftPorts' CLIError adopts the protocol in a follow-up PR.

Tests

commandFailurePrintsItsOwnDiagnosticAndStatus checks that the diagnostic is printed byte for byte and the exit status is 128. It fails without the bridge change. All 67 tests pass, and lint is clean.

🤖 Generated with Claude Code

[odrobnik]

@codex review

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-18T16:31:01.523421Z	48c4b8a	Manual request

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[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Swish!

Reviewed commit: 48c4b8a07f

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