Refine project documentation and track metric files

Close gaps found in a review of the documentation:
- Track data/metrics/ and data/metric_sources.json in git so the data
  checker passes on a fresh clone (finding 23; decision logged)
- State the Alaska and Hawaii coverage gap in the README limitations and
  extend finding 12
- File findings 24-26: the data-sources doc lacks gridMET and several
  pipeline commands; wettest/driest month are computed twice; solar GHI
  is written by the extreme-temperature apply step
- Add the stale "fallback values" note to filter 2's tasks

Tidy the document system:
- Add docs/reviews/README.md with the numbering rules and a finding index
- Rename koppen-mixed-display-plan.md to koppen-mixed-display.md and fix
  its stale Puerto Rico and "stage 5" text
- Add the precipitation-month step to the README enrichment list
- Describe the Current method / Previous method pattern in plan section 5
- Add CLAUDE.md with the project guardrails and doc layout

Format filter-calculations.md so it renders on GitHub and in VS Code:
inline math uses $...$, ranges use en dashes, and implementation
references name functions instead of line numbers.

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

docs/reviews/02-annual-avg-temperature.md

@@ -76,6 +76,9 @@ Climate Normals*). Alaska and Hawaii stay blank; see
 - [ ] Correct `scripts/county_data_sources.md`: Source 2 says the build reads
       monthly normals files, but it reads the nClimGrid monthly series and
       averages 1991–2020. Update the `avgTempF` definition line to match.
+      Also remove the "Run the generator" note that fallback values are
+      applied to counties outside NOAA coverage; they are left blank
+      (finding 12).
 - [ ] Add the area-weighted `avgTempF` to the §7 guardrail on rerunning
       `build_county_climate_data.py` in
       [pipeline-plan.md](../pipeline-plan.md).