Shorten generated branch name format to ambient/{session_id}

Changed auto-generated branch name from 'ambient-session-{uuid}' format
to simpler 'ambient/{session_id}' format. This makes branch names shorter
and more directly tied to the session ID.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: sallyom

components/runners/claude-code-runner/main.py

@@ -636,7 +636,8 @@ async def clone_repo_at_runtime(git_url: str, branch: str, name: str) -> tuple[b
 
     # Generate unique branch name if not specified
     if not branch or branch.strip() == "":
-        branch = f"ambient-session-{uuid.uuid4().hex[:8]}"
+        session_id = os.getenv("SESSION_ID", "unknown")
+        branch = f"ambient/{session_id}"
         logger.info(f"No branch specified, generated: {branch}")
 
     # Repos are stored in /workspace/repos/{name} (matching hydrate.sh)