chore: upgrade Pi SDK to 0.84.3

[benedict2310]

Summary

• upgrade @earendil-works/pi-agent-core, pi-ai, and pi-coding-agent from 0.82.1 to 0.84.3
• align TelePi's direct Undici dependency with Pi 0.84.3 at the exact reviewed version 8.9.0
• retain an explicit compatibility contract covering supported Node, all direct Pi package versions, Undici, and external extension imports

Compatibility assessment

• Pi 0.83/0.84 breaking changes concern TypeBox aliases, model-refresh/provider APIs, and harness/repository APIs; TelePi's documented root-package imports and SDK session lifecycle compile and pass compatibility/lifecycle coverage unchanged.
• Pi 0.84.3 keeps Node >=22.19.0, matching TelePi.

Validation

• Characterization before upgrade: npx vitest run test/pi-sdk-compatibility.test.ts test/pi-session.test.ts (84 passed)
• TDD RED: upgraded manifest failed the explicit 0.82.1 compatibility assertion; adding the exact Undici contract also RED before pinning 8.9.0.
• npx vitest run test/pi-sdk-compatibility.test.ts test/pi-session.test.ts (84 passed)
• npm test (441 passed, 1 Node-26-only test skipped locally on Node 23.11)
• npm run test:coverage (passed; statements/lines 86.63%, branches 80.98%, functions 90.09%)
• npm run build