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/skill.rs

@@ -183,6 +183,11 @@ impl SkillService {
                     return Ok(custom.join("skills"));
                 }
             }
+            AppType::OpenCode => {
+                if let Some(custom) = crate::settings::get_opencode_override_dir() {
+                    return Ok(custom.join("skills"));
+                }
+            }
         }
 
         // 默认路径：回退到用户主目录下的标准位置
@@ -196,6 +201,7 @@ impl SkillService {
             AppType::Claude => home.join(".claude").join("skills"),
             AppType::Codex => home.join(".codex").join("skills"),
             AppType::Gemini => home.join(".gemini").join("skills"),
+            AppType::OpenCode => home.join(".config").join("opencode").join("skills"),
         })
     }
 
@@ -425,6 +431,7 @@ impl SkillService {
                     AppType::Claude => "claude",
                     AppType::Codex => "codex",
                     AppType::Gemini => "gemini",
+                    AppType::OpenCode => "opencode",
                 };
 
                 unmanaged
@@ -468,6 +475,7 @@ impl SkillService {
                             AppType::Claude => "claude",
                             AppType::Codex => "codex",
                             AppType::Gemini => "gemini",
+                            AppType::OpenCode => "opencode",
                         };
                         found_in.push(app_str.to_string());
                     }