fix: revert WASM artifact SHA256 checksums to null (#627)

Reverts the checksums added in fe4c3c5. The baked-in checksums cause
production failures when the host binary's WIT version doesn't match
the artifacts at /releases/latest/ — WASM tools (web-search) and
channels (telegram) fail with "matching implementation was not found
in the linker".

Setting sha256 back to null unblocks the runtime install path
(ExtensionManager doesn't validate checksums) and allows the next
release-plz run to publish matching host + artifact pairs.

[skip-regression-check]

Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>

registry/tools/slack.json

@@ -18,7 +18,7 @@
   "artifacts": {
     "wasm32-wasip2": {
       "url": "https://github.com/nearai/ironclaw/releases/latest/download/slack-tool-wasm32-wasip2.tar.gz",
-      "sha256": "600fdb6f25f42bd635d3cf28217778c780e781b780c7250a57bebcf889616209"
+      "sha256": null
     }
   },
   "auth_summary": {