feat: add Nastech-Agent as a fourth runtime

[nastechresearch]

Summary

• Add Nastech-Agent as a fourth remote runtime target.
• Add secure bearer API-key support to connection profiles.
• Add runtime selection to both remote connection flows.
• Implement Nastech health, sessions, messages, models, durable runs, and stop requests through the real Nastech API.
• Add MockWebServer coverage for health authentication and run creation.

Design

AndCode remains the Android client. Nastech-Agent remains the source of truth for its Python runtime, tools, skills, memory, MCP, dashboard, gateway, Telegram, and background services. This PR implements the remote-first slice described in the integration plan and does not create a reduced Android-side Nastech implementation.

Validation

• ./gradlew spotlessCheck --no-daemon passes.
• git diff --check passes.
• Old com.yugahashimoto.andcode namespace is absent.
• Android compilation could not run in this sandbox because no Android SDK is installed; GitHub Actions should perform the full Android build and tests.