refactor: reorganize aish package structure

[F16shen]

Summary

• reorganize core modules into clearer package boundaries under llm, state, terminal, shell/commands, and scripts/themes
• restore root-level cli.py, config.py, system_info.py, sandboxd.py, and keep prompts.py at the package root
• realign packaging and test layout so imports, build paths, and spec files match the new structure

Details

• move provider and session code under src/aish/llm and move runtime/session helpers under src/aish/state
• move PTY and interaction code under src/aish/terminal while keeping shell-specific pieces under src/aish/shell
• rename scripts/prompts to scripts/themes and reorganize tests by feature area
• update Makefile, aish.spec, pyproject.toml, and affected imports for the new module layout

Testing

• /home/lixin/workspace/aishell/aish/.venv/bin/python -m pytest
  • 486 passed, 15 skipped

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Contribution guide: https://github.com/AI-Shell-Team/aish/blob/main/CONTRIBUTING.md

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