Consolidate docs/decisions.md (1923 -> 1028) by extracting four cohesive
topic clusters into docs/decisions/ (optimistic-concurrency,
api-auth-security, release-ci-governance, spa-modularization) — content
relocated verbatim (lossless; all rationale + reversals preserved), main
Index rebuilt to reference the topic files plus the remaining in-file
entries, docs/README.md points back at the decisions Index. Add the
v26.9.0 row to the ci-cd.md version table.
refs #340
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>