docs(matrix): move migration guide from install/ to channels/

The Matrix migration guide is plugin-upgrade content (encrypted-state recovery,
device verification, room-key restore) rather than a cross-system import or
machine move, so it belongs alongside the Matrix channel docs rather than under
Install > Maintenance > Migrating.

- Move docs/install/migrating-matrix.md to docs/channels/matrix-migration.md
- Update inbound link in docs/channels/matrix.md
- Update the migrating.md hub: replace the Matrix Card with a one-line link in 'Upgrade a plugin in place'
- Refresh Related list on the moved page (link Matrix push rules and Migration guide hub)
- docs.json: remove install/migrating-matrix from Maintenance > Migrating, slot channels/matrix-migration between channels/matrix and channels/matrix-push-rules in the Mainstream channels group, and add a /install/migrating-matrix -> /channels/matrix-migration redirect

docs/channels/matrix.md

@@ -401,7 +401,7 @@ Without `--account <id>`, Matrix CLI commands use the implicit default account.
 
     Startup also runs a conservative crypto bootstrap pass that reuses the current secret storage and cross-signing identity. If bootstrap state is broken, OpenClaw attempts a guarded repair even without `channels.matrix.password`; if the homeserver requires password UIA, startup logs a warning and stays non-fatal. Already-owner-signed devices are preserved.
 
-    See [Matrix migration](/install/migrating-matrix) for the full upgrade flow.
+    See [Matrix migration](/channels/matrix-migration) for the full upgrade flow.
 
   </Accordion>