fix(work verify): terminalize Ctrl-C cancellation

[solomonneas]

Summary

• Catch Ctrl-C at the brigade work verify run boundary, terminate the verifier child process group, and write a terminal receipt instead of leaving a running receipt or printing a traceback.
• Add receipt status canceled (command status interrupted, exit code 130) with preserved stdout/stderr, timing, git, and graph evidence where available.
• Record verify + canceled as a neutral outcome signal (0), distinct from engineering failed (-1).

Closes #441

Test plan

• brigade work verify run --target . --command "pytest -q tests/test_work_cmd_verification.py::test_work_verify_run_terminalizes_keyboard_interrupt tests/test_work_cmd_verification.py::test_run_verify_child_process_catches_keyboard_interrupt tests/test_work_cmd_verification.py::test_work_verify_run_canceled_status_is_neutral_not_a_regression_signal tests/test_outcome.py::test_signal_value_rewards_only_model_unauthored_success" --capture brigade-work
• brigade work verify run --target . --command "./scripts/verify" --capture brigade-work

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[greptile-apps]

Greptile Summary

This PR makes verification cancellation produce a terminal, neutral outcome. The main changes are:

• Runs verifier children in separate process groups and terminates them on Ctrl-C.
• Writes canceled receipts with preserved logs and available evidence.
• Returns exit code 130 without printing a traceback.
• Records canceled verification as a neutral outcome signal.

Confidence Score: 5/5

No additional blocking issue qualifies for this review round.

• The latest changes address cancellation, terminal receipt state, and neutral outcome handling.
• No distinct additional failure requiring a separate fix was identified.

Important Files Changed

Filename	Overview
src/brigade/work_cmd/verification.py	Adds child-process cancellation, output decoding, and centralized terminal receipt finalization.
src/brigade/outcome.py	Maps canceled verification receipts to a neutral outcome signal.
tests/test_work_cmd_verification.py	Adds coverage for cancellation receipts, child termination, and neutral outcome capture.
tests/test_outcome.py	Covers the canceled verification signal mapping.

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