Support JSONC (JSON with Comments) for settings.json

### What do you want to change? Allow ~/.pi/agent/settings.json and .pi/settings.json to be parsed as JSONC (JSON with // and /* */ comments, and optionally trailing commas), instead of strict JSON ### Why? Currently, settings files are documented and enforced as strict JSON, so users cannot annotate their configuration (e.g., explaining why a PackageSource entry is disabled, or documenting thinkingBudgets/enabledModels choices). JSONC support would let users maintain self-documenting, team-shareable project settings (especially relevant since` .pi/settings.json` is meant to be shared with a team per the packages docs) without needing an external annotated copy. ### How? (optional) _No response_