[Go SDK] Port .harness() to Go SDK

[santoshkumarradha]

Overview

Port the .harness() feature to the Go SDK. All 4 providers use CLI subprocess (no native Go SDKs exist for any coding agent).

Branch: TBD (separate from feat/harness-v2)
Design doc: docs/design/harness-v2-design.md (Sections 4, 8.3)

Scope

Go SDK (sdk/go/harness/)

• config.go — HarnessConfig struct
• types.go — HarnessResult, RawResult, Metrics types
• provider.go — Provider interface
• factory.go — BuildProvider() function
• runner.go — Runner with retry + schema orchestration
• schema.go — Schema handling (JSON Schema from struct tags)
• cli.go — Shared CLI subprocess utilities
• claude.go — Claude Code provider (CLI-based: claude -p)
• codex.go — Codex provider (CLI-based: codex exec)
• gemini.go — Gemini provider (CLI-based)
• opencode.go — OpenCode provider (CLI-based)
• Wire into Go Agent struct
• Tests for all components

Notes

• Go has no native SDKs for any coding agent → all providers are CLI subprocess
• Schema validation via json.Unmarshal with struct tags
• Follow patterns from Python/TS implementation
• This is a separate effort after Python + TS are complete and validated

Dependencies

• Depends on: Python + TS implementation being complete and merged (feat/harness-v2 PR)
• Reference: Epic issue for full context

Acceptance Criteria

• Feature parity with Python/TS implementation
• All 4 providers work via CLI subprocess
• Schema handling uses same file-write strategy
• All tests pass
• go test ./... clean

[github-actions]

Thanks for picking this up, @santoshkumarradha! 🎉

Here's what to expect:

• We'll check in after 7 days if we haven't heard an update
• After 14 days, we'll send a second reminder
• After 21 days without activity, we'll unassign to let others contribute

Tips for success:

• Drop a comment when you start working on it
• Share any blockers - we're here to help!
• Link your PR with Fixes #207 in the description

Happy coding! 🚀