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/mcp/claude.rs

@@ -91,6 +91,7 @@ pub fn import_from_claude(config: &mut MultiAppConfig) -> Result<usize, AppError
                         claude: true,
                         codex: false,
                         gemini: false,
+                        opencode: false,
                     },
                     description: None,
                     homepage: None,

src-tauri/src/mcp/codex.rs

@@ -235,6 +235,7 @@ pub fn import_from_codex(config: &mut MultiAppConfig) -> Result<usize, AppError>
                             claude: false,
                             codex: true,
                             gemini: false,
+                            opencode: false,
                         },
                         description: None,
                         homepage: None,

src-tauri/src/mcp/gemini.rs

@@ -87,6 +87,7 @@ pub fn import_from_gemini(config: &mut MultiAppConfig) -> Result<usize, AppError
                         claude: false,
                         codex: false,
                         gemini: true,
+                        opencode: false,
                     },
                     description: None,
                     homepage: None,