feat(worker): gate queued runs through scheduler admission

[dporkka]

Summary

Stacked on #58. Integrate the bounded scheduler into worker run admission without duplicating scheduling logic in the worker.

Behavior

• adds a fail-closed RunAdmission seam before ExecuteRun
• composes budget start admission before scheduler admission
• adds budget.Engine.CheckRunStart so a queued run is denied when active concurrency is already equal to the configured maximum, while preserving existing CheckRun semantics for already-running agents
• reads per-task scheduler claims from task.metadata.scheduler:
  • owns
  • cpu
  • memory_mb
• namespaces ownership by repository
• missing scheduler metadata conservatively claims the whole repository
• blocked JetStream messages are NAKed with a configurable delay rather than executed or acknowledged
• worker defaults capacity from host CPU plus configurable memory/max-parallel environment variables

Verification

• packages/scheduler: go test -race ./... — pass
• apps/api: go test ./internal/budget ./internal/agentrunner ./pkg/agentexecutor — pass
• apps/worker: go test ./internal/handlers ./cmd/worker — pass
• targeted API + worker go vet — pass
• make build builds API, CLI, worker, runner and TypeScript SDK successfully; web build then stops because the isolated validation environment does not have the Next.js binary installed
• make lint-go could not run in the isolated validation environment because golangci-lint is not installed

Follow-up

This PR adds safe admission, not execution parallelism. The current worker event callback invokes long-running ExecuteRun synchronously. The next change should add a bounded execution pool that reserves admission atomically before launching work, rather than spawning unbounded goroutines.

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[dporkka]

Superseded by the landed scheduler path. PR #60/#63 established atomic ownership/resource admission and task-spec ownership; PR #65 ported the remaining useful budget-start and deferred-redelivery behavior onto current main, with concurrency accounting based on atomic admitting claims. Closing this competing implementation to avoid maintaining two scheduler admission paths.