Add LESSONS_LEARNED.md for all 28 build repos

[Copilot]

Adds a lessons/ directory to the 286-builds meta-repo containing one detailed LESSONS_LEARNED.md per build, covering what worked, challenges, key insights, what to do differently next time, skills gained, and cross-build integration points.

Structure

lessons/
├── expense-tracker/LESSONS_LEARNED.md
├── Async-News-Aggregator/LESSONS_LEARNED.md
├── ... (28 total, named to match each repo)
└── github-dashboard-react-query/LESSONS_LEARNED.md

Each file (~450–650 words) follows a consistent six-section format:

• What Worked Well — patterns and libraries that delivered
• Challenges Overcome — specific problems + exact solutions
• Key Insights — architectural decisions worth remembering
• Next Time — concrete improvements
• Skills Gained — specific tools and competencies
• Integration Points — cross-build connections (e.g. CSV pipeline from Expense Tracker → Sales Dashboard, parallel-fetch pattern from Async News Aggregator → GitHub Dashboard, state management arc across Kanban → Typed Task Manager → Zustand Cart)

Note on individual repo PRs

The environment's GitHub token does not have write access to the 28 individual build repos. All content is consolidated here; copy each file to the corresponding repo root to open per-repo PRs, or create them directly via the GitHub UI.