feat: Add user research workflow to compound-engineering plugin

[mattthomps1]

Summary

Add a user research workflow that makes user interviews and personas a first-class input to AI-assisted development. Today the plugin has zero research capabilities — insights from user interviews sit in Google Docs and never reach the developer.

Proposed Components

• /workflows:research command — Orchestrates three phases: Plan research, Process transcripts, Build personas
• research-plan skill — Create structured research plans with discussion guides (Teresa Torres / Mom Test methodology)
• transcript-insights skill — Process interview transcripts into structured snapshots with tagged insights
• persona-builder skill — Synthesize insights across interviews into living persona documents with confidence tracking
• user-research-analyst agent — Surface relevant personas and insights during feature planning

Delivery

• PR 1: Research workflow, skills, agent, and directory structure (docs/research/)
• PR 2: Integration with /workflows:brainstorm and /workflows:plan to auto-surface research

References

• Brainstorm: docs/brainstorms/2026-02-10-user-research-workflow-brainstorm.md
• Plan: docs/plans/2026-02-11-feat-user-research-workflow-plan.md