fix: resolve install.sh latest version without the GitHub API (#325) (#336)

The standalone installer resolved the latest release via the GitHub API, which
rate-limits unauthenticated requests to 60/hr per IP and returns 403 on shared
or cloud hosts (devboxes, CI) — leaving "could not resolve latest version". It
now reads the version from the releases/latest web redirect (no rate limit),
falling back to the API, and normalizes CODEGRAPH_VERSION so a bare "0.9.4"
works as well as "v0.9.4".

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>