Add usage metering plus tier limits and concurrency controls

[BunsDev]

Per ROADMAP M4. Make familiar execution meterable and boundable so it can be charged for.

• Meter usage by installation, repo, familiar, and task runtime.
• Enforce tier limits (task caps, concurrency).
• Concurrency controls per installation/tier.
• Emit metering events to the audit/task store.

Depends on persistent task store (#2) and worker isolation (#5).