Warn dstack skill about heredocs split across commands items

[r4victor]

dstack joins commands items into one shell line with && after stripping each item:

dstack/src/dstack/_internal/server/services/jobs/configurators/base.py

Lines 600 to 606 in 0d578c8

	def _join_shell_commands(commands: List[str]) -> str:
	for i, cmd in enumerate(commands):
	cmd = cmd.strip()
	if cmd.endswith("&"): # escape background command
	cmd = "{ %s }" % cmd
	commands[i] = cmd
	return " && ".join(commands)

So a heredoc that ends an item never sees its terminator: the next item lands on the terminator line, the rest of the script becomes heredoc text, and the run exits 0 without running anything after it. A trailing # comment in an item silently drops the following items the same way.

This hit a preset creation session for Qwen3.8-Flash-Next on 4xH200: the agent split a heredoc writing the nginx config into its own commands item, so the final verification service exited 0 after the model download without starting the engines. The agent took about 10 minutes to notice and relaunch, which delayed the preset by ~10 minutes of idle 4xH200 time plus a second service launch.

• Add a best-practice bullet to the dstack skill (also bundled into the preset agent) to keep multi-line shell such as heredocs inside one commands item and to avoid trailing comments.

The underlying joining in _join_shell_commands is unchanged; making it heredoc-safe is a separate fix.