feat(monetization): implement auto-upsell trigger at 50% free credit threshold (Fixes #3)

[agent-operator-bot]

Objective

Closes #3 — Implements auto-upsell trigger that fires at the 50% free credit threshold (call #5 of 10).

Changes

• src/monetization/upsell.ts: Core checkUpsellTrigger() function with idempotent INSERT ON CONFLICT, A/B prompt variants, system_events logging, and Express middleware factory
• tests/upsell.test.ts: 8 Deno tests covering threshold detection, idempotency (no double-trigger), A/B determinism, multi-user independence, and event logging

Acceptance Criteria Met

• Middleware checks call count and sets X-Upsell-Prompt header
• A/B test ready prompt variants (deterministic per user)
• Fires exactly once per threshold crossing (idempotent)
• Tests for happy path + duplicate prevention

Contributor Wallet

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