fix(cli): abort headless streams on SIGINT

[d3f4w2]

Description

Fixes the headless CLI cancellation gap reported in #12701.

The active provider stream now receives an abort signal synchronously when the process receives SIGINT. This prevents delayed provider chunks from reaching tool-call handling while the process-level shutdown handler is still awaiting cleanup. The temporary signal listener is removed after both successful and failed requests.

The TUI signal path is unchanged.

Closes #12701.

AI assistance disclosure: I used an AI coding assistant to help trace the cancellation flow and prepare the patch. I reviewed the changes and ran the validation below.

AI Code Review

• Team members only: AI review runs automatically when PR is opened or marked ready for review
• Team members can also trigger a review by commenting @continue-review

Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Screen recording or screenshot

Not applicable; this fixes headless process cancellation behavior.

Tests

• npm test -- --run src/util/withSigintAbort.test.ts src/stream/streamChatResponse.test.ts — 10 passed, 6 skipped
• Full CLI suite — 1,673 passed, 49 skipped; one unrelated TUI timing assertion failed once and then passed 27/27 on an isolated rerun
• npm run build
• ESLint and Prettier checks on all changed files
• git diff --check