splashboard

Implement a new fetcher or renderer for splashboard. Use when the user asks to add a fetcher (e.g. `system_battery`, `weather_*`, `git_*`) or a renderer (e.g. `gauge_battery`, `chart_radar`). Drives the workflow: pick-candidate → spec-out → code → self-review → user smoke-tests via `.splashboard/dashboard.toml` → wrap up.