feat(core): continue subagent sessions - #40931

Summary

• add an optional sessionID input to continue an existing foreground subagent session
• preserve the child session history while validating parent ownership and agent identity
• expose the reusable session ID in foreground and background completion envelopes as sessionID
• reject running or explicitly backgrounded continuation until that lifecycle is implemented separately

Testing

• bun test test/tool-subagent.test.ts from packages/core
• bun typecheck from packages/core
• scoped Effect pattern scan for the changed Core source files

Requested by: @rekram1-node (Aiden Cline via Slack)