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/05-heat-index-days.md

@@ -0,0 +1,30 @@
+# Filter 5: 90 °F+ heat-index days
+
+**Status:** Not started.
+
+**Data keys:** `humidHeatDays`, with the audit columns `humidHeatSourceFips`
+and `humidHeatFipsAdjustment`. Calculation:
+[filter-calculations.md](../filter-calculations.md) §5.
+
+## Findings
+
+**7. Heat Index days are a daily-extrema proxy.** Daily Tmax and daily minimum
+relative humidity are paired even though their observation times may differ.
+The result should not be described as an observed hourly maximum Heat Index.
+
+**8. Heat-year completeness is permissive.** Any year with at least one valid
+Tmax/RH pair is included in the equal-year average. A minimum valid-day rule
+would reduce low-biased partial-year counts.
+
+Cross-filter findings that affect this filter, in
+[00-cross-filter.md](00-cross-filter.md): 12 (Alaska and Hawaii not covered)
+and 13 (Lexington, VA uses Rockbridge County as a proxy).
+
+## Decisions
+
+None yet.
+
+## Tasks
+
+Not started; follow the per-filter review checklist in
+[pipeline-plan.md](../pipeline-plan.md) §5.