plan status PR rollups and implemented-flip suggestion

Surface the traceability data structurally so missing PRs and ready-to-flip specs are visible without grepping prose.

Tracking started Jul 9 — check back for trends.

Relationships

main current draft

plan status includes a per-spec "PRs" rollup listing the unique (deduped) PRs across its implemented-by work items
plan status suggests the implemented flip when every implemented-by work item of an accepted spec is done
Rollups dedupe when one PR closes multiple work items
--format json output includes the rollup data
Tests cover the rollup, dedupe, and flip-suggestion logic