Add verified winners and baseline setup

[269394628]

What changed

• require a candidate to be fully verified before it can become the winner
• run verification once against the base commit and save the baseline result
• add repeatable worktree setup commands through config and CLI flags
• reject setup commands that fail or contaminate candidate patches
• terminate POSIX process groups on timeout
• clarify that configured agent providers may receive repository context
• add integration and configuration coverage for the new behavior

Why

A non-empty patch could previously be selected as the winner even when its configured checks failed. Detached worktrees also had no supported dependency setup stage, and results could not distinguish pre-existing test failures from candidate regressions.

Impact

PatchLeague now reports no winner unless every configured check passes. Existing configurations remain valid because the new setup stage defaults to an empty list. Users can opt into setup commands such as npm ci or uv sync --frozen.

Validation

• npm run verify
• 22 JavaScript files passed syntax checks
• 16/16 automated tests passed
• npm pack --dry-run
• git diff --check