The idx_one_pending_task_per_issue index only allowed one pending task
per issue across all agents, causing different agents' queued/dispatched
tasks to block each other. This mismatched the code-level dedup which
checks per (issue_id, agent_id). Replace with idx_one_pending_task_per_issue_agent
on (issue_id, agent_id) so each agent can independently have one pending task.
Fixes MUL-495
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
