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>

data/metric_sources.json

@@ -0,0 +1,4 @@
+{
+  "schemaVersion": 1,
+  "metrics": {}
+}