Mastra Adds Typed Signal Queueing

This is a small API addition with outsized value for production agents: structured events no longer need to masquerade as user prompts.

• –Preserves reactive or notification semantics across queued turns
• –Supports background inputs from CI, GitHub, and other external systems
• –Keeps queued context separate from ordinary user follow-ups
• –Adds signal provenance to `AGENT_RUN` spans for traceability