Retire the External checks tab in favor of the PolicyEngine scorecard

Decision (David): the reform-validation / External checks surface duplicates what policyengine-scorecard now does canonically ("a level is a score of the null reform" — our tab is mode 2 of its design). One home for external comparisons; this dashboard stays focused on calibration fit.

Moved, not lost: all five committed override artifacts (f0af251 → Build O) are ingested into scorecard_db as per-release pe_results history — PolicyEngine/policyengine-scorecard#15 (design) / PolicyEngine/policyengine-scorecard#16 (adapter, 222 claims / 530 results).

What goes (PR incoming): /populace/reforms + its API routes and view, reform-overrides/ + gen script, the 6-hourly backfill workflow (OOM-dead on the 7GB runner since 7/21 anyway), tools/reform_validation/. Nav entry becomes an external link to the scorecard.

What stays: the staging leg — reforms.ts parsing + the staging view's candidate reform_validation.json section (release-gate telemetry produced by populace itself). The staging view loses only the published-release side-by-side column, whose data source was the removed route.

Fate of #112: open but on hold — its Modal app + liveness machinery (reviewed by @PavelMakarchuk) is the natural per-release producer to re-home in the scorecard once the automation question in scorecard #15 is settled.

🤖 Generated with Claude Code

Review the /populace/reforms routes and view, reform-overrides/, its generation script, tools/reform_validation/, and the 6-hourly backfill workflow. Remove the published-release surface and change the navigation to link to the PolicyEngine scorecard while retaining reforms.ts parsing and the staging view's candidate reform_validation.json section.