Support unlimited concurrency limits

[yvette-carlisle]

Summary

• allow execution.max_concurrent_agents to be omitted or set to "unlimited"
• keep positive integer limits compatible while making capacity checks and dispatch-slot locks support unlimited mode
• update workflow docs/runtime notes for the new default

Validation

• cargo make check
• cargo run -p decodex --bin decodex -- status --config ~/.codex/decodex/projects//project.toml --json --limit 1 for all local Decodex projects

Local config

Updated all ~/.codex/decodex/projects/*/WORKFLOW.md max_concurrent_agents entries to "unlimited".