feat: improve reddit session reliability

[777genius]

Summary:

• Add Reddit whoami and headed login helper commands
• Allow REDDIT_HEADLESS, REDDIT_PROFILE_DIR, and REDDIT_BROWSER_EXECUTABLE overrides
• Validate the logged-in account through Reddit API state before posting
• Add an old Reddit API fallback for top-level comments when the UI login state is unreliable
• Add public visibility checks for newly posted comments

Tests:

• python3 -m py_compile scripts/reddit_browser.py
• git diff --check
• staged diff reviewed for local config, accounts, tokens, cookies, and product-specific data