fix(auto-capture): reuse injected opencode client

[huilang021x]

Summary

• What changed: Auto-capture now builds its structured-output session client from the injected OpenCode ctx.client instead of creating a second client from ctx.serverUrl. session.create failures now include underlying status/error details when available.
• Why: In no-port OpenCode sessions, ctx.serverUrl can be http://localhost:4096/ even when no REST API is listening there, causing session.create returned no session id during auto-capture.
• Current phase: Ready for review. Closes Auto-capture uses unreachable ctx.serverUrl in no-port OpenCode sessions #145.

Scope

• In scope: Structured-output auto-capture client initialization and regression coverage for the injected-client adapter.
• Out of scope: Provider/model timeout tuning, storage/indexing behavior, web UI behavior.
• Risk level: Low/medium; changes the internal client used for auto-capture session create/prompt/delete while preserving the existing request shapes.

Deployment Targets

• Impacted services: opencode-mem plugin auto-capture and user-profile structured-output paths that use the shared opencode provider client.
• Restart/deploy needed: Users need to restart OpenCode after upgrading the plugin.
• Target environment: OpenCode plugin runtime.
• Rollback note: Revert this commit to restore the previous ctx.serverUrl client path.

Acceptance Plan

• Acceptance criteria: Auto-capture should not fail with no session id solely because ctx.serverUrl points to an unreachable default port; structured-output calls should use the injected client and still pass format, system, model, and parts correctly.
• Evidence to collect: Regression test verifies createStructuredOutputClient() maps v2-style calls onto the injected client without opening a new serverUrl client.
• Required checks: Typecheck, build, formatter check, targeted provider tests, full test suite.
• Manual verification needed: Optional validation in a no-port OpenCode runtime after publishing.

Validation

• Commands run:
  • bun test tests/opencode-provider.test.ts
  • bun run format:check
  • bun run typecheck
  • bun run build
  • bun test
• Result: All passed after formatting.
• Evidence links/log snippets:
  • Targeted test: 12 pass, 0 fail
  • Full test suite: 157 pass, 0 fail