Add Power BI context source

[luca-martial]

Scope

Build a powerbi context-source adapter that ingests datasets, DAX measures, and report metadata from Power BI workspaces.

Where to start

• Adapter directory: packages/context/src/ingest/adapters/powerbi/
• Model on packages/context/src/ingest/adapters/looker/
• Use the Power BI REST API; service principal auth via Azure AD
• Wire the new driver value in packages/context/src/project/driver-schemas.ts
• Add docs in docs-site/content/docs/integrations/context-sources.mdx

Acceptance criteria

• New adapter under adapters/powerbi/ with tests
• Ingest reads dataset schema and DAX measures from a configured workspace
• Output writes semantic-layer YAML and/or wiki Markdown
• Authentication uses Azure AD service principal via env: references
• Integration docs added

Reward

Merging this PR earns the hoodie tier (tier-3). See CONTRIBUTING.md.

Comment on this issue to claim it before starting work.