fix(core): remove orphaned packages/core left by the #1050 squash-merge (#1120)

#1050 (Phase C) deleted packages/core in favor of @meshtastic/sdk, but the
squash-merge hit a delete/modify conflict with #1097 (which had modified
packages/core/package.json and src/meshDevice.ts), leaving those two files
behind as a broken stub. Its package.json still declared a tsdown build
(entry: "mod.ts") while mod.ts and the rest of the source were gone, so the
'Push to Main CI' build-and-package loop failed with:
  Error: Cannot find entry: "mod.ts"

Nothing depends on the workspace @meshtastic/core — the app resolves
@meshtastic/core to the JSR-published @jsr/meshtastic__core@2.6.6 — so finish
the deletion and drop the now-unused pnpm-lock importer.

Verified locally: frozen-lockfile install passes and the full build-and-package
loop builds every package green (sdk, sdk-react, sdk-storage-sqlocal, all
transports).

pnpm-lock.yaml

@@ -317,15 +317,6 @@ importers:
         specifier: ^4.1.8
         version: 4.1.8(@types/node@24.12.0)(happy-dom@20.8.9)(jsdom@25.0.1)(vite@7.3.5(@types/node@24.12.0)(jiti@2.7.0)(lightningcss@1.31.1)(terser@5.48.0)(tsx@4.21.0)(yaml@2.8.2))
 
-  packages/core:
-    dependencies:
-      '@meshtastic/protobufs':
-        specifier: workspace:*
-        version: link:../protobufs
-      crc:
-        specifier: npm:crc@^4.3.2
-        version: 4.3.2
-
   packages/protobufs:
     dependencies:
       '@bufbuild/protobuf':