CLI: Document exit codes in --help output

[titouanmathis]

Context

The CLI has a well-defined exit code hierarchy in packages/cli/src/error-handler.ts:

Code	Meaning
0	Success
1	General error
2	Authentication error
3	Validation error
4	Configuration error
5	Not found

These are useful for agents and scripts to branch on failure type without parsing error messages. However, they're not documented anywhere user-facing.

Proposal

Add an EXIT CODES section to the main productive --help output:

EXIT CODES:
  0    Success
  1    General error (unexpected failure)
  2    Authentication error (invalid/expired token)
  3    Validation error (missing or invalid arguments)
  4    Configuration error (missing apiToken, orgId, etc.)
  5    Resource not found

Also mention in the README.