Update DEVELOPMENT_PLAN.md with TopicBriefing documentation

[AndreRobitaille]

Context

The TopicBriefing feature (#64) added a new model, three-tier generation pipeline, editorial voice guidelines, and updated the topic show page. Per project conventions, docs/DEVELOPMENT_PLAN.md should be updated to reflect these additions.

What to document

• TopicBriefing in Core Domain Model section (new model, one-per-topic, three generation tiers)
• Three-tier generation pipeline in the workflow/architecture section
• Editorial voice reference (point to AUDIENCE.md editorial voice section)
• New jobs: Topics::UpdateTopicBriefingJob, Topics::GenerateTopicBriefingJob in Job Namespaces
• New rake task: briefings:generate in the commands table
• Updated topic show page structure (headline → editorial → record)

Files

• docs/DEVELOPMENT_PLAN.md — needs updates
• docs/plans/2026-02-21-topic-briefing-architecture-design.md — reference for what was built

[AndreRobitaille]

Closing — DEVELOPMENT_PLAN.md already documents TopicBriefing: Core Domain Model entry, three-tier generation pipeline table, two-pass architecture (analyze → render), editorial voice guidelines, citation rules, and updated topic show page structure. CLAUDE.md also fully documents the model, jobs, and homepage card architecture.