Refactor reviewer-bot module imports

[PLeVasseur]

Summary

• stop routing json, os, and yaml through the reviewer-bot module namespace
• import those modules directly where they are used in extracted reviewer-bot helpers
• keep the change narrow and validate the affected runtime paths with focused tests

What Changed

• update lease-lock helpers to use direct json imports instead of bot.json
• update automation helpers to use direct os imports instead of bot.os
• update the show-state maintenance path to use a direct yaml import instead of bot.yaml
• remove the corresponding compatibility-only imports from scripts/reviewer_bot.py
• adjust focused reviewer-bot regression tests to reflect the reduced runtime facade

Validation

• uv run python -m pytest .github/reviewer-bot-tests/test_main.py .github/reviewer-bot-tests/test_reviewer_bot.py
• uv run ruff check --fix scripts .github/reviewer-bot-tests/test_main.py .github/reviewer-bot-tests/test_reviewer_bot.py

Stacking

• first PR in the reviewer-bot facade reduction series

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