Re-add integration tests to CI

[marcusrbrown]

Context

Integration tests were removed from CI in #36 to unblock plugin-loading isolation work. They run locally only today.

What needs to happen

• Decide which integration scenarios are worth running on every PR vs only on workflow_dispatch.
• Re-add the integration job to .github/workflows/ci.yaml with if: github.event_name != 'push' so it runs on PRs and manual dispatch but not on every push to main.
• Re-add Integration tests to the required-status-check contexts in .github/settings.yml for main.
• Confirm the chosen test model (currently opencode/minimax-m2.5 paid by default; OPENCODE_TEST_MODEL overrides) does not blow through Copilot quota under PR load.

Why this is parked

The free models (opencode/big-pickle, opencode/minimax-m2.5-free) hit https://opencode.ai/zen rate limits (HTTP 429) under repeated runs and silently retry until tests time out. Local runs use a paid model; that 's not appropriate to default in CI without first scoping cost.

Done when

• Integration job runs on PRs and manual dispatch.
• Required status check is back on main.
• A short note in AGENTS.md documents the model-override env var and the cost characteristic.