When a subagent was spawned while the parent had runtime-modified
settings (fast mode, thinking, etc.), the subagent would start with
defaults from `AgentSettings` instead of inheriting the parent's current
values.

This adds `inherit_parent_settings` as a single place in
`Thread::new_subagent` that copies all runtime-mutable settings from the
parent:

- `speed`
- `thinking_enabled`
- `thinking_effort`
- `summarization_model`
- `profile_id`

These are the same properties that `set_*` methods already propagate to
`running_subagents` — this fix ensures they're also inherited at
creation time.

cc @benbrandt

Release Notes:

- N/A