Add unified agent sync providers for credentials and preferences

[gricha]

Summary

• Introduces a unified AgentSyncProvider interface for syncing agent credentials and preferences to workspaces
• Creates a central registry at src/agents/ that combines sync and session providers for each coding agent
• Replaces hardcoded sync methods in WorkspaceManager with the new abstraction
• Adds comprehensive unit tests with mock FileCopier for testability without Docker

Architecture

src/agents/
├── types.ts              # Core interfaces (AgentSyncProvider, SyncFile, etc.)
├── index.ts              # Central registry + syncAllAgents orchestrator
├── sync/
│   ├── types.ts          # FileCopier interface
│   ├── copier.ts         # Docker + mock implementations
│   ├── claude-code.ts    # Claude Code sync provider
│   ├── opencode.ts       # OpenCode sync provider
│   └── codex.ts          # Codex sync provider
└── __tests__/            # Unit tests (40 tests, no Docker required)

What Each Agent Now Syncs

Agent	Credentials	Preferences
Claude Code	~/.claude/.credentials.json	settings.json, CLAUDE.md, ~/.claude/agents/ (skills), MCP servers from ~/.claude.json
OpenCode	Generated opencode.json with zen_token	MCP config from host's opencode.json
Codex	~/.codex/auth.json	~/.codex/config.toml

Key Design Decisions

1. Testability first: FileCopier interface allows unit testing without Docker
2. One-time sync: Credentials/preferences sync on workspace create/start (not continuous)
3. Central registry: src/agents/index.ts is the single entry point for all agent capabilities
4. Option C architecture: Keeps existing session providers in place, adds sync providers alongside

Changes

• New: src/agents/ - Agent sync providers and central registry
• New: test/integration/agent-sync.test.ts - Integration test
• Modified: src/workspace/manager.ts - Replaced 3 methods with syncAllAgents() call
• Modified: src/agent/file-watcher.ts - Uses getCredentialFilePaths() from agents module
• Modified: test/helpers/agent.ts - Fixed agents config passthrough

Testing

• 40 new unit tests for providers and orchestrator
• 4 new integration tests for agent sync
• All 189 existing tests pass