feat(reviewer-bot): add reviewer board preview groundwork

[PLeVasseur]

Summary

• add reviewer-board Patch 1 preflight and read-only preview support, including GraphQL project metadata discovery and desired-value derivation
• expose the preview through the sweeper workflow as a manual Patch 1.5 smoke-test path with targeted issue input and board token wiring
• add reviewer-bot coverage for read-only preview behavior, workflow exposure, and date-stable overdue expectations

Testing

• uv run pytest .github/reviewer-bot-tests/test_main.py .github/reviewer-bot-tests/test_reviewer_bot.py

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