Implements Issue #249 - Let Timmy query his own thinking.
Changes:
- Add search_thoughts() function to thinking.py that queries the thoughts DB
- Support filtering by query string (content LIKE matching)
- Support optional seed_type filter
- Support configurable limit (default 10, max 50)
- Returns formatted string with timestamps and seed types
- Register thought_search tool in tools.py via _register_thinking_tools()
- Add thought_search to the tool catalog
- Add 7 comprehensive tests for the new functionality
The tool enables Timmy to reflect on his past thoughts, building the
foundation for self-awareness and continuity of thinking across sessions.
Fixes#249
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