Add per-agent maxConcurrentRuns (1-10) controlling how many runs
execute simultaneously. Implements agent-level start lock, optimistic
claim-then-execute flow, atomic token accounting via SQL expressions,
and proper status resolution when parallel runs finish. Updates UI
config form, live run count display, and SSE invalidation to avoid
unnecessary refetches on run event streams.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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