feat(chaoss): Phase 6 — 6 more Level-0 metrics (contributor cadence + PR lifecycle)

Total goes from 22 → 28 metrics. Contributor 4 → 6, Lifecycle 10 → 14.

Contributor cadence:

  inactive_contributors     Distinct authors whose most-recent commit
                            predates the window. Mirrors the "dormant"
                            bucket from project_demographics as a
                            standalone metric. Donut tone goes
                            danger when inactive share > 70 %.

  occasional_contributors   Distinct authors with ≤ 4 commits in the
                            window — CHAOSS's drive-by threshold.
                            For gimie · 5y: 6 of 16 active contribs.

PR lifecycle (use github_*_enriched, light up on stacks with the
GrimoireLab GitHub backend ingested):

  cr_accepted               Merged PRs in window (acceptance half of
                            closure_ratio). Monthly sparkline.

  cr_declined               Closed-not-merged PRs. The other half;
                            pair with cr_accepted for accept/reject
                            split. Monthly sparkline.

  cr_duration               P50 / P90 days from PR open to MERGE for
                            PRs that shipped. Acceptance-speed view.

  pr_time_to_close          P50 / P90 days from PR open to CLOSE for
                            any closed PR (merged + declined).
                            Differs from cr_duration: a wide gap
                            between the two means the project closes-
                            without-merging slowly.

All six ship with python / bash / js recipes via _build_recipes; two
shared helpers (_pr_count_metric, _pr_percentile_metric) capture the
boilerplate for the four PR-lifecycle metrics.

Author: caviri