fix: reuse authenticated OpenCode client transport

[catan271]

Summary

• Reuse the OpenCode host SDK transport, including custom fetch and headers, when creating the v2 client.
• Prefer the authenticated SDK session methods for structured output when available.
• Retain the raw HTTP fallback for older SDK builds.
• Add regression coverage for host headers and response content types.

Validation

• bun test tests/opencode-provider.test.ts tests/plugin-host-config.test.ts
• bun run typecheck
• bunx prettier --check src/index.ts src/services/ai/opencode-host-config.ts src/services/ai/opencode-provider.ts src/services/ai/opencode-sdk-client.ts tests/opencode-provider.test.ts tests/plugin-host-config.test.ts

Closes #174

[tickernelz]

Merged into main and included in the v2.20.0 release tag. Verified on the fully integrated batch tree with bun install --frozen-lockfile, bun run typecheck, bun run build, and bun test (226 pass / 0 fail). Thanks for the contribution!