docs(usage): add --debug example + exit codes to help text (#171)

The 'testsprite usage' command help text listed three examples but
omitted the --debug flag (a global option useful for diagnosing auth
and network issues). It also didn't document the exit codes, which
matters for CI/CD scripts that gate on the return value.

This PR adds:
- A --debug example showing what it traces (HTTP method/path, request
  id, latency) — useful when debugging 'auth error' or 'transport
  failure' messages.
- An explicit exit-codes section (0 success, 3 auth error, 10 network
  failure) so users scripting against the CLI know what to expect.

Pure documentation improvement — no behavioral change, no new deps.
Tested: existing unit tests pass (npm test).

Co-authored-by: MOAAMN SAYED <moaamnsayed560@gmail.com>

Author: mo01115285816-cyber

src/commands/usage.ts

@@ -200,7 +200,12 @@ export function createUsageCommand(deps: UsageDeps = {}): Command {
       '\nExamples:\n' +
         '  testsprite usage                 # show balance + plan\n' +
         '  testsprite usage --output json   # machine-readable balance\n' +
+        '  testsprite usage --debug         # trace HTTP method/path, request id, latency\n' +
         '  testsprite credits               # alias for usage\n' +
+        '\nExit codes:\n' +
+        '  0   success (or --dry-run)\n' +
+        '  3   auth error — run `testsprite setup` to configure credentials\n' +
+        '  10  transport/network failure (UNAVAILABLE) — retry the command\n' +
         '\nNote: credit balance requires a backend update to /me. Until shipped,\n' +
         "  check your portal's Billing page (/dashboard/settings/billing) for your balance.",
     )