Proactive fix suggestions from the terminal/dev-server error stream #253

Inspired by Warp Code's proactive suggested diffs.

Watch the thread's embedded terminal and dev-server output for compiler errors, test failures and merge conflicts; when one appears, offer a fix unprompted at the failure site (accept / dismiss / reprompt / fork a thread from the suggestion). Distinct from #161 (CI/review reaction loop): this is local, live, and unsolicited.