Set VERSION 1.18.0 and fold the changelog into [1.18.0] before the pithead 2.0.0 pin #565

Owner ruling on pithead#2602 (2026-10-02): the next RigForge release is 1.18.0. The pithead 2.0.0 image pins the develop commit that carries it (pithead#3029), and the release is cut after the 2.0.0 soak. VERSION and the changelog land on develop now, so that after the soak the release is only the promotion and the tag (rigforge#564). At most the changelog date moves.

• VERSION: change 1.17.4 to 1.18.0.
• CHANGELOG.md: remove the never-released [1.17.4] - 2026-09-08 section. Fold its Fail closed across the combined appliance release gates #491 entry and every change since v1.17.3 into [1.18.0].

This is not a release: no tag, no GitHub release, no promotion to main. Do not block this on rigforge#564; that tracker waits on this commit.