fix: prevent subagent model changes from persisting to parent

[elithrar]

When viewing a subagent session, the model sync was persisting to the parent session's model store, causing model "bleed" back to the main agent.

This PR has OpenCode skip the model/agent sync for sessions with parentID (subagent sessions)

Note: Resolved conflict with #6528 which added a partial fix for agent sync only; this PR extends it to also skip model/variant sync.

Fixes #6636