ci(release): let release-please bump server.json's version itself (#9874)

check-server-manifest requires server.json's version and packages[0].version to
equal @loopover/mcp's, and release-please bumped the package without touching
server.json -- so every generated release PR failed CI until someone synced the file
by hand.

A manual fix does not hold: #9780 was fixed that way and release-please regenerated
the branch and discarded the commit, putting server.json back to 3.15.2. That is the
whole reason this belongs in the config rather than in a script someone runs.

The leading slash is load-bearing. release-please joins an extra-file path to the
package directory UNLESS it starts with "/" (strategies/base.js addPath), so plain
"server.json" would have resolved to packages/loopover-mcp/server.json and silently
updated nothing. Verified against the installed release-please 17.10.4: "/server.json"
resolves to the repo root, and the GenericJson updater rewrites both jsonpaths in the
real file.

Author: JSONbored

release-please-config.json

@@ -4,7 +4,11 @@
     "packages/loopover-mcp": {
       "release-type": "node",
       "component": "mcp",
-      "package-name": "@loopover/mcp"
+      "package-name": "@loopover/mcp",
+      "extra-files": [
+        { "type": "json", "path": "/server.json", "jsonpath": "$.version" },
+        { "type": "json", "path": "/server.json", "jsonpath": "$.packages[0].version" }
+      ]
     },
     "packages/loopover-engine": {
       "release-type": "node",