chore(local): improve runnable cmds preambles in sg start (#64339)

`sg start ...` commands have a preamble field to inform the user about
various things. Prior to this PR, they were rather easy to miss. Along
the way, I've fixed the printing so if there multiple lines of preamble,
they appear nicely.

This PR addresses that. 


<details><summary>before/after</summary>
<p>
<img
src="https://github.com/user-attachments/assets/22d94ebb-e247-4e4e-8dab-1f502f1e8b46"/>
<img
src="https://github.com/user-attachments/assets/7cbbf41b-a926-4ebd-9f6d-bbdd779cc8b4"/>
</p>
</details> 

## Test plan

<!-- REQUIRED; info at
https://docs-legacy.sourcegraph.com/dev/background-information/testing_principles
-->

Locally tested, see before/after.

Author: jhchabran

dev/sg/internal/run/command.go

@@ -298,7 +298,13 @@ func startSgCmd(ctx context.Context, cmd SGConfigCommand, parentEnv map[string]s
 	}
 
 	if conf.Preamble != "" {
-		std.Out.WriteLine(output.Styledf(output.StyleOrange, "[%s] %s %s", conf.Name, output.EmojiInfo, conf.Preamble))
+		// White on purple'ish gray, to make it noticeable, but not burning everyone eyes.
+		preambleStyle := output.CombineStyles(output.Bg256Color(60), output.Fg256Color(255))
+		lines := strings.Split(conf.Preamble, "\n")
+		for _, line := range lines {
+			// Pad with 16 chars, so it matches the other commands prefixes.
+			std.Out.WriteLine(output.Styledf(preambleStyle, "[%-16s] %s %s", fmt.Sprintf("📣 %s", conf.Name), output.EmojiInfo, line))
+		}
 	}
 
 	return startCmd(ctx, opts)