feat(blacksmith-cache): add BLACKSMITH_BYPASS_CHECKOUT control-plane kill switch (#29)

Introduces shouldUseBlacksmithCache(), which honors the new
BLACKSMITH_BYPASS_CHECKOUT env var (driven by the per-installation
`bypass_blacksmith_checkout` flag in the control plane). When set to
"true", the action skips every Blacksmith-specific code path (mirror
cache setup, alternates, dissociate, post-step cache commit) and
behaves identically to upstream actions/checkout.

git-source-provider.ts swaps its single isBlacksmithEnvironment() gate
for the new helper. Because the post-step cleanup gates on state values
that are only set inside that block, the post step automatically
becomes a no-op when the kill switch fires - no other changes required.

isBlacksmithEnvironment() is left untouched so its name continues to
match its meaning (am I running on a Blacksmith VM).

Includes jest coverage for the env x kill-switch matrix and a
rebuilt dist/index.js.

Made-with: Cursor