Add modernization workflow canvas (#2758)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

extensions/modernization-workflow/README.md

@@ -0,0 +1,26 @@
+# Modernization Workflow canvas
+
+An interactive GitHub Copilot CLI canvas that visualizes the documented
+Modernize CLI lifecycle:
+
+1. **Assess** the application and generate evidence.
+2. **Plan** a reviewable modernization strategy and task list.
+3. **Execute** transformations with build, security, and result validation.
+
+The canvas detects .NET, Java, and C++ repository signals, generates commands
+from the current workflow configuration, and refreshes progress from artifacts
+under `.github/modernize/`.
+
+## Canvas ID
+
+`modernization-workflow`
+
+## Agent actions
+
+- `get_workflow_state`
+- `configure_workflow`
+- `set_step_status`
+- `refresh_from_artifacts`
+
+Workflow state is stored in `.github/modernize/canvas-<stateId>.json` in the
+application repository so progress survives canvas and CLI restarts.