src/cli/upgrade.ts — self-update

Purpose

vx upgrade [tag] downloads the release asset for this os/arch and atomically renames it over the current executable.

Invariants

• Compiled-binary detection keys off Bun.main / process.argv[1] (/$bunfs/…), NOT import.meta.path — under --minify --bytecode the latter reports the original source path (the 2026-06-15 bug).
• Running from source refuses with a git-pull hint.
• isBunfsPath(p) exported for tests.