Create a TSP file starter document (Azure#3351)

Fixes Azure#1882 

@ronniegeraghty for detailed commentary.

Largely created by copilot with substantive edits after the fact.

Copilot prompt tested successfully on "chatgpt 5 Codex", mostly
successfully on "Claude Sonnet 3.5"

---------

Co-authored-by: Copilot <[email protected]>
Co-authored-by: Heath Stewart <[email protected]>

Author: 3 people

.github/prompts/perf-test.prompt.md

@@ -1,5 +1,5 @@
 ---
-mode: "agent"
+agent: "agent"
 description: "Generate a new performance test"
 ---
 

.vscode/settings.json

@@ -13,7 +13,10 @@
     }
   ],
   "markdownlint.config": {
-    "MD024": false
+    "MD024": false,
+    "MD046": {
+      "style": "consistent"
+    }
   },
   "rust-analyzer.cargo.features": "all",
   "rust-analyzer.check.command": "clippy",