feat(opencode): Phase 1 - Backend data structure expansion for OpenCode support

Add OpenCode as the 4th supported application with additive provider management:

- Add OpenCode variant to AppType enum with all related match statements
- Add enabled_opencode field to McpApps and SkillApps structures
- Add opencode field to McpRoot and PromptRoot
- Add database schema migration v3→v4 with enabled_opencode columns
- Add settings.rs support for opencode_config_dir and current_provider_opencode
- Create opencode_config.rs module for config file I/O operations
- Update all services (proxy, mcp, skill, provider, stream_check) for OpenCode
- Add OpenCode support to deeplink provider and MCP parsing
- Update commands/config.rs for OpenCode config status and paths

Key design decisions:
- OpenCode uses additive mode (no is_current needed, no proxy support)
- Config path: ~/.config/opencode/opencode.json
- MCP format: stdio→local, sse/http→remote conversion planned
- Stream check returns error (not yet implemented for OpenCode)

src-tauri/src/services/config.rs

@@ -122,6 +122,10 @@ impl ConfigService {
             AppType::Codex => Self::sync_codex_live(config, &current_id, &provider)?,
             AppType::Claude => Self::sync_claude_live(config, &current_id, &provider)?,
             AppType::Gemini => Self::sync_gemini_live(config, &current_id, &provider)?,
+            AppType::OpenCode => {
+                // OpenCode uses additive mode, no live sync needed
+                // OpenCode providers are managed directly in the config file
+            }
         }
 
         Ok(())