Standardize exit codes across all CLI commands

[betegon]

Context

PR review comment from @AustinBYK: the exit code pattern used by sentry init is great and should be spread across the entire CLI, with documentation on the docs site.

Current state

• Only sentry init uses structured exit codes; every other command exits 0 or 1
• CliError base class (src/lib/errors.ts) has an exitCode property but no subclass overrides it — all errors default to 1
• Init wizard defines 4 constants in src/lib/init/constants.ts (10, 20, 30, 50) with 2 more used server-side only (40, 41)

Existing init exit codes

Code	Meaning	Where defined
0	Success	open-sentry-ui step
10	Sentry already installed	check-existing-sentry step
20	Platform not detected	detect-platform step
30	Dependency install failed	install-deps step
40	Codemod plan failed	plan-codemods step (server-side only)
41	Codemod apply failed	apply-codemods step (server-side only)
50	User stopped after verification	verify-changes step

Proposal

1. Define semantic exit code ranges for the whole CLI (e.g. 1=general, 2=usage/args, 10–19=auth, 20–29=config, etc.)
2. Have CliError subclasses set appropriate exit codes (e.g. AuthError → 10, ConfigError → 20)
3. Add a reference page to the docs site listing all exit codes so users can script around them
4. Backward-compatible — currently everything is 0 or 1, so adding specific codes is purely additive