feat: add exemplar session library

[ccf]

Summary

• add a workflow-aware exemplar session library to Growth pattern sharing
• enrich pattern-sharing responses with exemplar workflow, cost, and linked-pattern evidence
• update the roadmap to mark shipped workflow items and capture website positioning follow-up

Testing

• pytest tests/test_growth.py -q
• cd frontend && ./node_modules/.bin/vitest run --run src/components/growth/tests/bright-spot-cards.test.tsx src/components/growth/tests/exemplar-session-library.test.tsx src/pages/tests/growth.test.tsx

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Note

Medium Risk
Adds new derived data to the pattern-sharing analytics response (including workflow and cost enrichment) and surfaces it in the Growth UI, which could affect query performance and downstream consumers expecting the prior response shape.

Overview
Adds an Exemplar Session Library to the Growth "Patterns" tab, rendering workflow-aware exemplar cards (support, success, duration, cost, tools, linked patterns) with an empty-state fallback.

Extends the pattern-sharing analytics response to include exemplar_sessions, derived from each cluster’s best approach and enriched with SessionWorkflowProfile, SessionFacets.brief_summary, tool usage, and estimated cost computed from aggregated ModelUsage token totals.

Updates backend/frontend API schemas and adds unit/integration tests covering the new response fields and UI rendering; roadmap is updated to mark shipped items and add a website positioning follow-up section.

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