Release & versioning structure (VERSION + tags + CHANGELOG + process)

[VijitSingh97]

Goal

Give RigForge a real release process — it has no VERSION, no tags-as-releases, no CHANGELOG. Mirrors pithead#44, scaled to a script-based tool.

Proposed

• Top-level VERSION (semver) as the single source of truth; surfaced by --version/doctor.
• Tagged GitHub releases + a hand-curated CHANGELOG.md.
• Independent versioning from the stack (per the repo split), with a documented "known-good pithead/stack version" compatibility note.
• The release records the pinned XMRIG_VERSION (ties to the pinned-build issue) as its ingredients manifest.
• (Eventually the immutable image — rigforge#1, v2 — becomes the pullable artifact.)

Acceptance

• VERSION + --version; tagged release with CHANGELOG entry.
• Documented release steps + stack-compatibility note.