Smarter, faster generations via per-request model routing

Every generation now runs through a lightweight router that decides (a) whether to send the whole project or just the files relevant to your prompt, and (b) whether a smaller, faster model can handle trivial single-line or cosmetic edits. Small edits return more quickly and cost less, while complex changes still get the full-power model and full project context. The router's choice and reason are surfaced in the chat activity timeline (e.g. "Editing 3 file(s)" or "Editing whole project — app is small"), with an "Easy Mode" suffix when the lighter model is used.