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/09-driest-month.md

@@ -7,13 +7,13 @@
 
 ## 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).
+No filter-specific findings yet.
 
 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).
+[00-cross-filter.md](00-cross-filter.md): 12 (Alaska and Hawaii not covered),
+16 (the app's nClimGrid source link returns 404), 24 (no precipitation-month
+command in the data-sources doc), and 25 (computed in both the base build and
+the precipitation-month script).
 
 ## Decisions