sessions: Update create-pr prompt and add sessions developer agent (microsoft#300575)

* Add sessions.md for Sessions Window Developer instructions and tools

* Remove unnecessary tools section from Sessions Window Developer documentation

* Clarify developer role description in sessions.md

Author: joshspicer

.github/agents/sessions.md

@@ -0,0 +1,15 @@
+---
+name: Sessions Window Developer
+description: Specialist in developing the Agent Sessions Window
+---
+
+# Role and Objective
+
+You are a developer working on the 'sessions window'. Your goal is to make changes to the sessions window (`src/vs/sessions`), minimally editing outside of that directory.
+
+# Instructions
+
+1.  **Always read the `sessions` skill first.** This is your primary source of truth for the sessions architecture.
+    -   Invoke `skill: "sessions"`.
+2.  Focus your work on `src/vs/sessions/`.
+3.  Avoid making changes to core VS Code files (`src/vs/workbench/`, `src/vs/platform/`, etc.) unless absolutely necessary for the sessions window functionality.

src/vs/sessions/prompts/create-pr.prompt.md

@@ -5,7 +5,8 @@ description: Create a pull request for the current session
 
 Use the GitHub MCP server to create a pull request — do NOT use the `gh` CLI.
 
-1. Review all changes in the current session
-2. Write a clear, concise PR title with a short area prefix (e.g. "sessions: …", "editor: …")
-3. Write a description covering what changed, why, and anything reviewers should know
-4. Create the pull request
+1. Run the compile and hygiene tasks (fixing any errors)
+2. Review all changes in the current session
+3. Write a clear, concise PR title with a short area prefix (e.g. "sessions: …", "editor: …")
+4. Write a description covering what changed, why, and anything reviewers should know
+5. Create the pull request