docs(hermes): rework CLI migrate page and add user-facing migration guide

- cli/migrate: convert flat reference into structured Mintlify page (Tip pointer, ParamField for flags, AccordionGroup for safety model, sub-sections for Hermes provider with what's imported, .env keys, archive-only state, and plugin contract)
- install/migrating-hermes: new dedicated user guide modeled after migrating-matrix.md (Tabs for onboarding vs CLI, AccordionGroup for what gets imported, Steps for recommended flow, Warning for --overwrite, Troubleshooting accordions)
- docs.json: add install/migrating-hermes to Maintenance group alongside migrating and migrating-matrix

docs/docs.json

@@ -1012,6 +1012,7 @@
                 "pages": [
                   "install/updating",
                   "install/migrating",
+                  "install/migrating-hermes",
                   "install/migrating-matrix",
                   "install/uninstall",
                   "install/development-channels"