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/09-driest-month.md

@@ -0,0 +1,25 @@
+# Filter 9: Driest month
+
+**Status:** Not started.
+
+**Data key:** `driestPrecipMonth`. Calculation:
+[filter-calculations.md](../filter-calculations.md) §9.
+
+## Findings
+
+No numbered findings yet. Known issue to review: same as wettest month, which
+is computed in both the base build and the precipitation-month script; see
+[08-wettest-month.md](08-wettest-month.md).
+
+Cross-filter findings that affect this filter, in
+[00-cross-filter.md](00-cross-filter.md): 12 (Alaska and Hawaii not covered)
+and 16 (the app's nClimGrid source link returns 404).
+
+## Decisions
+
+None yet.
+
+## Tasks
+
+Not started; follow the per-filter review checklist in
+[pipeline-plan.md](../pipeline-plan.md) §5.