Add tool stubs for native agent tools
Stub implementations for:
- base.py: Tool, ToolResult, @tool decorator, registry
- calculate.py: Math expressions (simpleeval)
- fetch.py: HTTP requests (aiohttp)
- files.py: read_file, write_file, list_dir
- shell.py: run_command (sandboxed)
- search.py: web_search
- keyvalue.py: key_value_get/set/delete (in-memory stub)
- librarian.py: exist-db integration (store, get, query, search)
All stubs return "Not implemented" - ready for real implementation.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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