[codex] Add Power BI metadata connector

[abhijeethp]

Summary

• add a Power BI / Fabric scanner export connector that writes AGENTS.POWERBI_* tables
• implement Power BI as a normal source module (powerbi.py) following the existing dbt/LookML/OSI connector pattern
• add Power BI provider discovery rows in AGENTS.ROOT and expose agents-schema powerbi --metadata-path
• add first-class Power BI packaging: composite action, reusable workflow, setup guide, and example workflow
• add DuckDB as a local validation destination so real scanner exports can be tested without Snowflake
• keep existing dbt, Looker, and OSI connectors unchanged; non-PowerBI connector experiments were removed from the PR scope

Validation

• uv run python -m unittest discover -s tests
• uv run python -m compileall -q src tests
• uv run agents-schema --help
• uv run agents-schema powerbi --help
• Power BI fixture -> agents-schema powerbi --metadata-path ... -> DuckDB smoke test with row-count verification for agents.root and all agents.powerbi_* tables

Testing real Power BI exports

Set WAREHOUSE_CREDENTIALS to a local DuckDB file and run the connector against a Fabric / Power BI scanner scanResult JSON:

export WAREHOUSE_CREDENTIALS='{"type":"duckdb","path":"tmp/agents-schema.duckdb"}'
uv run agents-schema powerbi --metadata-path tmp/powerbi-scan-result.json