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).

packages/core/package.json

@@ -1,51 +0,0 @@
-{
-  "name": "@meshtastic/core",
-  "version": "2.6.7",
-  "description": "Core functionalities for Meshtastic web applications.",
-  "exports": {
-    ".": "./mod.ts"
-  },
-  "type": "module",
-  "main": "./dist/mod.js",
-  "module": "./dist/mod.js",
-  "types": "./dist/mod.d.ts",
-  "license": "GPL-3.0-only",
-  "tsdown": {
-    "entry": "mod.ts",
-    "platform": "browser",
-    "dts": true,
-    "format": [
-      "esm"
-    ],
-    "splitting": false,
-    "clean": true
-  },
-  "jsrInclude": [
-    "mod.ts",
-    "src",
-    "README.md",
-    "LICENSE"
-  ],
-  "jsrExclude": [
-    "src/**/*.test.ts"
-  ],
-  "files": [
-    "package.json",
-    "README.md",
-    "LICENSE",
-    "dist"
-  ],
-  "scripts": {
-    "preinstall": "npx only-allow pnpm",
-    "prepack": "cp ../../LICENSE ./LICENSE",
-    "clean": "rm -rf dist LICENSE",
-    "build:npm": "tsdown",
-    "publish:npm": "pnpm clean && pnpm build:npm && pnpm publish --access public --no-git-checks",
-    "prepare:jsr": "rm -rf dist && pnpm dlx pkg-to-jsr",
-    "publish:jsr": "pnpm run prepack && pnpm prepare:jsr && deno publish --allow-dirty --no-check"
-  },
-  "dependencies": {
-    "@meshtastic/protobufs": "workspace:*",
-    "crc": "npm:crc@^4.3.2"
-  }
-}