Story: Document the sprint health charts

Every sprint page carries four health charts (PRs & commits per day, daily line churn, PR cycle time, cumulative stories done), but nothing documents them as a set: what each chart measures, why we generate it, and how to read it. The four gnuplot scripts that render them sit in scripts/ as undocumented developer scripts, disconnected from the charts they produce.

This story delivers a single agile knowledge page that documents the charts — definition, measure, purpose, and interpretation, phrased in the project's agile domain terms — and becomes the literate home of the gnuplot sources, tangled back to scripts/ so compass sprint charts keeps working unchanged. The sprint Charts sections and the agile process page link to it.

• A knowledge page exists under doc/agile/ documenting all four sprint health charts: what each chart is, what it measures, its purpose, and how to interpret it (healthy and unhealthy patterns).
• The page uses the project's agile domain definitions (story, task, sprint, DONE, PR cycle) with id-links to their sources.
• The four gnuplot scripts live in the page as org-babel source blocks tangled to scripts/sprint_*.gnuplot; the tangled files carry a header marking them as generated from the page.
• compass sprint charts renders all four charts unchanged.
• The sprint Charts section (current sprint and the codegen sprint template) links to the page.
• The agile process page links to the page.
• The page links to the sprint health review runbook and to the compass recipe for generating the charts.

• Changing what the charts measure or how compass sprint charts extracts the metrics.
• Adding new chart types.