Plan: record the 2026-09-25 cleanup decisions (#45)

[bbstats]

Records the maintainer's 2026-09-25 decisions on the #45 sweep in the plan files:

• The leftover experiment folder is deleted, and so are the 5 merged remote branches.
• Random effects, slice 2: slopes, second grouping column, entity-ID auto-route #113 and Random effects? #109 are closed.
• The random-slopes archive branch is kept.
• The GPU hardware check and the KDD98 dataset are marked won't-do.
• refresh(X, y): fold new rows into a fitted model (#131, slice 1) #170 is being tested by hand.

The .gitignore line is in #175. Self-merged under the markdown rule.

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