fix(build): migrate pnpm external-tools entry to platforms schema

socket-registry moved the pnpm tool entry from a checksums map (asset +
raw sha256 hex) to a platforms map (asset + SRI integrity). Align
external-tools.json so consumers reading .pnpm.platforms[...] (the
fleet-canonical shape) resolve correctly. Integrity decodes to the same
sha256 the checksums block carried; assets and pinned version unchanged.

Author: jdalton

external-tools.json

@@ -29,32 +29,32 @@
       "notes": [
         "Required: yes",
         "Bootstrap locally via `corepack enable pnpm`",
-        "CI downloads + sha256-verifies the pinned tarball"
+        "CI downloads + integrity-verifies the pinned tarball"
       ],
-      "checksums": {
+      "platforms": {
         "darwin-arm64": {
           "asset": "pnpm-darwin-arm64.tar.gz",
-          "sha256": "32a50710ccacfdcf14e6d5995d5368298eec913b0ce3903b9e09b6555f06f4e5"
+          "integrity": "sha256-MqUHEMys/c8U5tWZXVNoKY7skTsM45A7ngm2VV8G9OU="
         },
         "darwin-x64": {
           "asset": "pnpm-darwin-x64.tar.gz",
-          "sha256": "71dca33f4275da6b43bf1eb40bdc4d876f59a116716eacbf01079c3d985ff85d"
+          "integrity": "sha256-cdyjP0J12mtDvx60C9xNh29ZoRZxbqy/AQecPZhf+F0="
         },
         "linux-arm64": {
           "asset": "pnpm-linux-arm64.tar.gz",
-          "sha256": "2dd04127ff10b1f9dd20bae248b779c77a8ec67e3afa35e7256e5f94abddd493"
+          "integrity": "sha256-LdBBJ/8QsfndILriSLd5x3qOxn46+jXnJW5flKvd1JM="
         },
         "linux-x64": {
           "asset": "pnpm-linux-x64.tar.gz",
-          "sha256": "7ebef4b616ba41fb0d54a207b36508fae3346723283a088b43fc1e038ee6fed0"
+          "integrity": "sha256-fr70tha6QfsNVKIHs2UI+uM0ZyMoOgiLQ/weA47m/tA="
         },
         "win-arm64": {
           "asset": "pnpm-win32-arm64.zip",
-          "sha256": "e4a39ad4c251db5e34b18b98561ef25bab5506ad65cad2fa3602af58d1972667"
+          "integrity": "sha256-5KOa1MJR2140sYuYVh7yW6tVBq1lytL6NgKvWNGXJmc="
         },
         "win-x64": {
           "asset": "pnpm-win32-x64.zip",
-          "sha256": "147485ae2f38c3d1ccf2f5db00d0244416bcd22b9114c02388e6a78f41538fc4"
+          "integrity": "sha256-FHSFri84w9HM8vXbANAkRBa80iuRFMAjiOanj0FTj8Q="
         }
       }
     },