docs: clarify first-run setup

[rohitmulani63-ops]

Closes #7
/claim #7

Summary

• Added a first-run checklist for starting from the repository root, preparing helpers/.env, installing dependencies, and running main.py.
• Clarified that the microphone selection is saved in mic_config.json and how to reset it if the wrong input device was chosen.

Validation

• Ran git diff --check.
• Not run: runtime microphone smoke test, because this is a README-only change.

[opirebot]

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