[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).

Design doc: docs/design/harness-v2-design.md (Sections 4, 8.3)

Scope

Go SDK (sdk/go/harness/)

• HarnessConfig struct (in sdk/go/agent/harness.go)
• result.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
• claudecode.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 (Agent.Harness() / Agent.HarnessRunner())
• Tests (runner_test.go, schema_test.go)

Remaining Work

• Codex provider (codex.go) — not yet implemented
• Gemini provider (gemini.go) — not yet implemented
• BuildProvider() factory (factory.go) — not yet implemented

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

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! 🚀

[github-actions]

Hey @santoshkumarradha! 👋

Just a friendly reminder that you're assigned to this issue. It's been about a week since assignment.

How's it going? We'd love to hear:

• Any progress updates?
• Running into any blockers we can help with?
• Need more time? (Totally fine!)

A quick comment helps us know you're still interested. If your plans have changed, no worries at all - just let us know and we can find another contributor.

Thanks for contributing to AgentField! 🚀

[github-actions]

Hi @santoshkumarradha,

Just checking in again! It's been 14 days since we last heard from you on this issue.

Quick options:

• 💬 Drop a comment with a progress update
• 🔗 Link a draft PR if you've started working on it
• 👋 Let us know if you'd like to be unassigned (no judgment!)

If we don't hear back in 7 days, we'll unassign this issue to give other contributors a chance to help.

Thanks for understanding!

[santoshkumarradha]

@AbirAbbas with your new pr this one can be closed right ?