feat: Add worker queues

[joshjon]

Summary

Adds named worker queues so that repositories can be routed to workers with specific tech stack tooling. This allows scaling across multiple repos with different stacks by running workers configured with different agent Docker images, each polling from a dedicated queue.

Changes

• Add queue column to repo table (default: "default"), so each repository can be assigned to a worker queue
• Add platform-level worker queue management via settings API (GET/POST/DELETE /settings/worker-queues); the "default" queue always exists and cannot be removed
• Add PUT /repos/:repo_id/queue endpoint to assign a repo to a queue, with validation that the queue exists
• Update poll endpoint (GET /agent/poll) to accept ?queue= parameter — resolves queue to repo IDs and filters tasks, epics, and conversations accordingly
• Add --queue / QUEUE flag to worker CLI (default: "default")
• Add Worker Queues section to Settings dialog for managing available queues
• Add queue selector dropdown to Repository Settings dialog
• Queue is stored on the repo, not the task — when a repo's queue changes, pending tasks automatically route to the new queue with no migration

[github-actions]

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