v0.10.0 — `scaffold.json` — manifest declaring scaffold version, core infrastructu

Added

• scaffold.json — manifest declaring scaffold version, core infrastructure files, skills, prompts, and removed files; lives in the template repo only — deleted by setup.sh during project setup
• bin/update — shell script that pulls scaffold infrastructure updates from the template repo; supports --yes (auto-accept new files) and --core-only (skip new skills/prompts) flags; uses proper semver comparison via awk; downloads to temp directory with atomic move; configurable upstream URL via .scaffold-upstream file; handles deprecated files via manifest removed array
• .scaffold-version tracking — setup.sh now reads the scaffold version from scaffold.json and writes it to .scaffold-version before deleting the manifest; bin/update updates .scaffold-version after successful updates
• "Keeping up to date" section in README.md documenting bin/update usage, update behavior categories, independent version tracks, and custom upstream configuration

Changed

• setup.sh — added .scaffold-version creation from scaffold.json, scaffold.json deletion, and bin/update to post-setup summary output
• README.md — added .scaffold-version and bin/update to file tree