cmd/vendorhash: track vendor SRI in flakehashes.json

Move the headscale vendorHash out of flake.nix into a content-
addressed flakehashes.json maintained by a small Go tool. The
schema and goModFingerprint algorithm mirror upstream tailscale's
tool/updateflakes so a future shared library extraction is trivial.

vendorhash check verifies flakehashes.json against the current
go.mod/go.sum. Hot path is a sha256 over those two files, so
re-runs without input change are essentially free; only an actual
fingerprint drift triggers go mod vendor + nardump.SRI.

vendorhash update recomputes both fields and rewrites the JSON.
The nix-vendor-sri devShell shim now wraps it.