feat: add SmolAgents integration with Hindsight memory tools

[benfrank241]

Summary

• Adds hindsight-smolagents package under hindsight-integrations/smolagents/
• Three native SmolAgents Tool subclasses: HindsightRetainTool, HindsightRecallTool, HindsightReflectTool
• create_hindsight_tools() factory and memory_instructions() helper for system prompt injection
• 81 unit tests, lint clean
• Docs page, sidebar entry, integrations grid entry, and HuggingFace icon

Design notes

• bank_id is baked in at construction time (SmolAgents has no RunContext equivalent)
• Sync forward() methods call the Hindsight Python client directly
• Follows the same config/error/tools module pattern as hindsight-agno

Test plan

• cd hindsight-integrations/smolagents && uv run pytest tests/ -v — all 81 tests pass
• uv run ruff check . && uv run ruff format --check . — lint clean
• Manual end-to-end: start API, retain/recall/reflect against a live bank
• Docs page renders and icon displays in integrations grid
• ./scripts/release-integration.sh smolagents recognizes the integration name