Upgrade path: `rigforge.sh upgrade`, idempotent re-runs, and prune old build archives

[VijitSingh97]

Problem

There's no upgrade story and re-running is wasteful/leaky:

• Every run re-clones + recompiles XMRig from scratch (minutes), even when nothing changed.
• prepare_workspace moves xmrig → xmrig-<timestamp> on every run and never cleans up (rigforge.sh:159-162) — archives accumulate forever (observed 8+ stale xmrig-* dirs on a test box). A disk leak.

Proposed

• An upgrade flow that re-pulls the pinned XMRIG_VERSION, rebuilds, and restarts the service only if the version changed; skip recompile when the pinned version is already built (idempotent re-runs).
• Prune old build archives (keep last N, or remove on successful build).
• A self-update note/command for the RigForge repo itself (git pull + re-run).

Acceptance

• Re-running with no version change is a fast no-op (no needless recompile).
• Old xmrig-* archives are pruned; disk doesn't grow unbounded.
• upgrade rebuilds + restarts only on a version bump.