Add promotional credit notifications and bump version

[aliroberts]

Add promotional credit notifications

Summary

• Prompt users to apply promotional credits when running a job with a bare model name (e.g. <model> → gemini/<model> if Gemini credits are available)
• Show a confirmation message when a prefixed model already matches an active grant
• Display promotional credit balances in weco credits balance output
• Version bump 0.3.15 → 0.3.16

How it works

• Before a run starts, check_promotional_credits() calls GET /billing/balance to check for active grants
• If the user specified a bare model name and has promotional credits, they're prompted to use the grant's provider prefix
• If the user already specified a provider prefix that matches a grant, a confirmation is shown with the remaining balance
• Skipped entirely for BYOK users (not billed through Weco)
• All balance API calls are best-effort with a 5s timeout — failures never block a run