Expand CI: run the test suite + shfmt (pin shellcheck)

[VijitSingh97]

Goal

.github/workflows/lint.yml only runs shellcheck on rigforge.sh + util/proposed-grub.sh. Broaden it to pithead's level.

Proposed

• Add a test job that runs the new suite (above).
• Add shfmt formatting check.
• Pin the shellcheck version (avoid the same apt-mirror/version flakiness pithead hit in #64 — install the static release tarball + checksum, or pin the action).
• (Optional, gated) a compile-smoke job that actually builds the pinned XMRig.

Acceptance

• CI runs shellcheck + shfmt + the test suite on every PR.
• shellcheck version pinned/reproducible.