Allows setting thinkingDefault on individual agent entries to override the global agents.defaults.thinkingDefault.
Priority order (highest to lowest):
1. Inline /think directive
2. Session-level thinking (persisted /think)
3. Agent-specific thinkingDefault (NEW)
4. Global agents.defaults.thinkingDefault
5. Model default
Example config:
```yaml
agents:
defaults:
thinkingDefault: low
list:
- id: code-reviewer
thinkingDefault: high # uses more reasoning for reviews
- id: quick-responder
thinkingDefault: off # fast responses
```
