docs: add Slack CLI setup instructions to README

[srtaalej]

Summary

• Adds collapsible "Using Slack CLI" section with install, login, create, and install commands
• Wraps existing manual setup in a collapsible "Using Terminal" section
• Adds a Development section with slack run and python3 app.py options
• Matches the pattern used by bolt-python-assistant-template and bolt-python-search-template

Test plan

• Verify rendered README displays both collapsible sections correctly
• Confirm CLI commands reference the correct template repo name