refactor: apply minimal tfcode branding

- Rename packages/opencode → packages/tfcode (directory only)
- Rename bin/opencode → bin/tfcode (CLI binary)
- Rename .opencode → .tfcode (config directory)
- Update package.json name and bin field
- Update config directory path references (.tfcode)
- Keep internal code references as 'opencode' for easy upstream sync
- Keep @opencode-ai/* workspace package names

This minimal branding approach allows clean merges from upstream
opencode repository while providing tfcode branding for users.

packages/tfcode/migration/20260225215848_workspace/migration.sql

@@ -0,0 +1,7 @@
+CREATE TABLE `workspace` (
+	`id` text PRIMARY KEY,
+	`branch` text,
+	`project_id` text NOT NULL,
+	`config` text NOT NULL,
+	CONSTRAINT `fk_workspace_project_id_project_id_fk` FOREIGN KEY (`project_id`) REFERENCES `project`(`id`) ON DELETE CASCADE
+);