fix: make team integration tests honor selected models

[codexstar69]

Summary

• make the worker integration harness accept an explicit test model so workers don't silently fall back to the global default
• make the RPC ready-handshake integration test load the teams extension so it matches the production spawn path
• add a changelog note for the test-harness/model-selection fix

Verification

• bun x eslint scripts/lib/pi-workers.ts scripts/integration-rpc-ready-handshake-test.mts
• PI_TEAMS_TEST_MODEL="amp-8317/gpt-5.4" bun run integration-rpc-ready-handshake-test
• PI_TEAMS_TEST_MODEL="amp-8317/gpt-5.4" bun run integration-claim-test -- --agents 2 --tasks 3 --timeoutSec 60