fix(help): hide ASCII banner when stdout is not a TTY (#501)

## Summary

Suppresses the SENTRY ASCII banner in `sentry` / `sentry help` when
stdout is piped or redirected. Interactive terminal sessions still show
the banner.

## Changes

Wraps the banner output in `printCustomHelp()` with a
`process.stdout.isTTY` check.

## Test plan

- `bun run dev` → banner visible
- `bun run dev | cat` → no banner
- `bun test test/lib/help.test.ts` → all 8 tests pass

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: betegon

src/lib/help.ts

@@ -109,10 +109,12 @@ export async function printCustomHelp(): Promise<string> {
 
   const lines: string[] = [];
 
-  // Banner with gradient
-  lines.push("");
-  lines.push(formatBanner());
-  lines.push("");
+  // Skip banner for non-TTY to avoid wasting tokens in agent loops
+  if (process.stdout.isTTY) {
+    lines.push("");
+    lines.push(formatBanner());
+    lines.push("");
+  }
 
   // Tagline
   lines.push(`  ${TAGLINE}`);