P1: Add CI workflow

[devcodex2025]

Source: docs/GROWTH_AND_AGENT_BACKLOG.md

Area: DevOps / quality
Priority: P1
Owner type: agent or human contributor
Files likely affected: .github/workflows

Goal:
Run install, lint, test, and build checks for root, web, and admin apps.

Acceptance criteria:

• Implementation matches the backlog goal.
• Tests are added or a clear reason is documented if tests are not applicable.
• User-facing errors or recommendations are actionable and do not leak secrets.
• Documentation/API notes are updated when behavior changes.

Risk:
Medium. Keep the change scoped and avoid unrelated refactors.