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>

scripts/apply_koppen_metric_to_climate_data.py

@@ -1,4 +1,4 @@
-"""Apply the county Koppen-Geiger metric to the app CSV.
+"""Apply the county Köppen-Geiger metric to the app CSV.
 
 Replaces the koppenZone column of data/climate-data.csv with the values in
 data/metrics/koppen.csv and writes koppenPrimaryClass and koppenSecondaryClass:
@@ -71,7 +71,7 @@ def load_koppen_values(koppen_metric: Path) -> Dict[str, Dict[str, str]]:
 def apply_koppen_metric(
     climate_data: Path, koppen_metric: Path, out: Path, dry_run: bool = False
 ) -> Tuple[List[Change], List[str]]:
-    """Update the Koppen columns; return the value changes and any columns that were added."""
+    """Update the Köppen columns; return the value changes and any columns that were added."""
     fields, rows = read_csv_rows(climate_data)
     if ZONE_FIELD not in fields:
         raise ValueError(f"{climate_data} has no {ZONE_FIELD} column.")
@@ -101,9 +101,9 @@ def apply_koppen_metric(
 
 def parse_args() -> argparse.Namespace:
     """Define and parse command-line options for this apply step."""
-    parser = argparse.ArgumentParser(description="Apply the county Koppen metric to the app CSV.")
+    parser = argparse.ArgumentParser(description="Apply the county Köppen metric to the app CSV.")
     parser.add_argument("--climate-data", type=Path, default=DEFAULT_CLIMATE_DATA, help="App climate CSV to update.")
-    parser.add_argument("--koppen-metric", type=Path, default=DEFAULT_KOPPEN_METRIC, help="Koppen metric CSV.")
+    parser.add_argument("--koppen-metric", type=Path, default=DEFAULT_KOPPEN_METRIC, help="Köppen metric CSV.")
     parser.add_argument("--out", type=Path, default=None, help="Output path; defaults to updating --climate-data in place.")
     parser.add_argument("--dry-run", action="store_true", help="Report changes without writing.")
     return parser.parse_args()