fix: disable Agent0 pull-request reviews

[mikkokotila]

What changed

• discard review_requested notifications before context fetch or scheduling
• mark ignored review requests read, while failing closed if that API call errors
• retain a second router guard that excludes review_requested
• preserve mention, assignment, comment, author, and CI-failure handling
• align specifications and operator documentation with the disabled review path

Why

A legacy zero-bang review path must not execute even if GitHub requests that account as a reviewer. This makes the retired behavior mechanically unreachable without affecting Agent0's other actions.

Validation

• uv run pytest -q — 242 passed
• uv run ruff check .
• uv run ruff format --check .
• uv run pyright — 0 errors

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