Implement ready queries and atomic eligible task claims #94

Work

Compute ready work from approved #76 semantics, with project/assignment filters, stable priority/FIFO order and paging. Recheck prerequisites and assignment during claim; prevent two agents from acquiring one task. Return clear conflict/not-ready results without private metadata.

Acceptance

• Completing a blocker makes dependent work discoverable; reopening blocks readiness again.
• Manual blocked and canceled-prerequisite cases behave as specified.
• Concurrent claim and dependency change cannot bypass eligibility.
• Own in-progress tasks remain discoverable separately.

Dependencies

Dependency persistence. Preserve existing version and parent-completion protections.