Fix platform-support issue template to apply a label that exists

The template auto-applies `platform-support`, but the repo has no such label
(harness requests use `new-harness`). GitHub silently drops labels that don't
exist, so every platform-support request arrives unlabeled — the Amazon Q
request (#2194) is the latest example.

Claude-Session: https://claude.ai/code/session_01UiEfXTZAC5cuH4hgx24mbB

.github/ISSUE_TEMPLATE/platform_support.md

@@ -1,7 +1,7 @@
 ---
 name: IDE / Platform Support Request
 about: Request support for a new IDE, editor, or AI coding tool
-labels: platform-support
+labels: new-harness
 ---
 
 <!--