chore(analysis): archive finished reviews, drop stale duplicates

Move the completed 2026-09 candidate review (MODLIST-REVIEW/-GROUPS,
CANDIDATE-APPENDIX, PACK-CONTEXT), FINDINGS.md and the shipped FTB Chunks swap
write-up into analysis/archive/, with a README mapping their old paths. The
review's build pieces, subagent intermediates and Modrinth API dumps are
deleted: the assembled reports carry their content. MODLIST-GROUPS.md was a
byte-for-byte duplicate of analysis/groups-table.md.

Also: drop the now-pointless FINDINGS/MODLIST packwizignore lines (analysis/
covers the archive), and gitignore *.tar.gz so the 650 MB map-migration world
tarball can't be committed by accident.

analysis/FTB-XAERO-MAP-MIGRATION.md

@@ -4,7 +4,7 @@
 and unit-tested; final step blocked on a copy of the server world save.
 **Goal:** give each player a native Xaero's map covering, as closely as possible, exactly the terrain
 they had revealed in FTB Chunks.
-Confidence tags as in `FINDINGS.md`.
+Confidence tags as in `analysis/archive/FINDINGS.md`.
 
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