feat: resize live workflow concurrency within the existing cap

[SSS135]

Summary

Adds mutable FIFO concurrency control for live workflow runs.

• workflow_control set_concurrency
• /workflows concurrency <id> <positive integer>
• requested and effective concurrency reporting
• scale-up releases queued work immediately in FIFO order
• scale-down lets active work finish while new starts honor the lower limit
• nested workflows share the same mutable limiter
• queued work checks cancellation before entering the running state
• requests use the existing supported range of 1..16

Temporary stack

This draft builds on #83 because live resizing extends its control and observability contract. Review the concurrency commit after #83, or wait until #83 lands and this branch is rebased.

Validation

• npm test: 909/909 passed across 64 suites
• focused concurrency and control suites: passed
• Biome: passed
• TypeScript build: passed
• git diff --check: passed
• independent shortcut, quality, and spec reviews completed; abort-before-start and requested/effective reporting findings were addressed and re-reviewed

Split from and supersedes the live-concurrency portion of #74.