Document restructuring and the beginnings of Filter 2 changes

Split the pipeline documentation by purpose so each fact has one home:
- docs/pipeline-plan.md keeps the plan, checklist, tracker, and guardrails
- docs/decisions.md holds open decisions and the dated decision log
- docs/reviews/ holds findings and tasks: one file per filter, plus
  00-cross-filter.md for findings that span filters
- scripts/common/README.md holds the shared-helper rules (formerly Phase 2)
- filter-calculations.md now describes calculations only

Filed findings 12-22 from a consistency audit of the app, docs, and scripts.

Filter 1 (Köppen-Geiger): use "Köppen" with the umlaut in all prose, labels,
docstrings, help text, and checker messages (finding 21), and correct the
base build's "majority" docstring (finding 22).

Filter 2 (annual avg temperature): record the adopted definition in
filter-calculations.md §2: equally weighted 1991-2020 monthly normals, per
WMO-No. 1203 and NOAA's 2020 methodology; area-weighted county means; blank
unless all 12 months exist. Code changes for this filter are still pending.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/reviews/12-clear-sky-ghi-reduction.md

@@ -0,0 +1,26 @@
+# Filter 12: Clear-sky GHI reduction
+
+**Status:** Not started.
+
+**Data key:** `clearSkyGhiReductionIndex`. Calculation:
+[filter-calculations.md](../filter-calculations.md) §12.
+
+## Findings
+
+**10. Clear-sky reduction averages ratios rather than energy totals.** This is
+a valid but specific definition. It gives each retained time row equal weight,
+rather than weighting rows by available clear-sky energy. The label and
+documentation should retain this distinction.
+
+Cross-filter findings that affect this filter, in
+[00-cross-filter.md](00-cross-filter.md): 11 (inconsistent aggregation) and
+16 (the app's NSRDB source link no longer resolves).
+
+## Decisions
+
+None yet.
+
+## Tasks
+
+Not started; follow the per-filter review checklist in
+[pipeline-plan.md](../pipeline-plan.md) §5.