feat: Add multi-provider code agent interface with Claude Code and OpenCode adapters
Implements weeks 1-4 of the multi-provider architecture:
Week 1 - Foundation:
- Add domain models (AgentProvider, AgentRequest, AgentEvent, AgentResult)
- Define CodeAgent port interface with Execute, Cancel, Capabilities
- Create thread-safe provider registry with first-registered default
Week 2 - Claude Code Adapter:
- Extract kubectl exec logic into CodeAgent implementation
- Parse stream-json output format (init, message, tool_use, result)
- Support session continuation via --resume flag
Week 3 - OpenCode Adapter:
- HTTP/SSE client for opencode serve API
- Session management (create, send message, abort)
- Event streaming with documented buffer rationale
Week 4 - Quality & Polish:
- Fix race condition in OpenCode Cancel method
- Add AgentRequest.Validate() with ErrPromptRequired, ErrInvalidTimeout
- Document DefaultAvailabilityTimeout constants
- Add HTTP error context for debugging
Also includes:
- Work queue system with PostgreSQL adapter
- Credential store for infrastructure secrets
- Project templates with Woodpecker CI integration
- Comprehensive test coverage
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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