Enforce AgentStack status jumps

[phucnguyen1707]

Summary

• add transition validation for AgentStack pending, queued, and running task states
• keep terminal states final and leave blocked re-entry behavior unchanged while the PRD open question remains unresolved
• cover invalid pending -> running and queued -> complete jumps in the AgentStack tests

Tests

• npm test --workspace @logicsrc/agentstack
• npm run build --workspace @logicsrc/agentstack
• git diff --check