chore: refactor --help and remove deprecated add alias

[Loongphy]

What changed

• Usage errors now produce actionable messages instead of silently showing help.
  Invalid invocations (unknown commands, missing values, duplicate flags) return exit code 2 with a specific error message pointing at the problem, rather than silently falling back to top-level help.

• Every command supports --help / -h with command-specific output.
  codex-auth list --help, codex-auth import --help, etc. now show focused Usage and Examples sections for that command. codex-auth help works too.

• add alias is gone.
  codex-auth add no longer works as a deprecated alias for codex-auth login. Use codex-auth login.

Note

Remove deprecated add alias for login and add command-specific --help support

• Removes the deprecated codex-auth add alias for codex-auth login from the CLI, docs, and all deprecation warning code paths.
• Refactors parseArgs in cli.zig to return a ParseResult union (either a parsed Command or a UsageError) instead of always defaulting to help on bad input.
• Adds command-scoped help pages: help <command> and <command> --help now render targeted usage and examples for each subcommand.
• Invalid CLI usage now prints a descriptive error with a usage hint to stderr and exits with code 2 (previously exited with code 1).
• Removes non-table output formats (json, csv, compact) from format.printAccounts in format.zig, which now always renders the table view.
• Behavioral Change: codex-auth add is no longer recognized as a valid command; callers using it will now receive a usage error instead of a deprecation warning.

^{Macroscope summarized 63ef466.}

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commit: 63ef466

[Loongphy]

@macroscope review

[macroscopeapp]

Just FYI for future @mentions, I'm Macroscope-App, not Macroscope.

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• Macroscope - Correctness Check

[macroscopeapp]

Approvability

Verdict: Would Approve

This is a CLI refactoring that removes a previously-deprecated command alias (add), restructures help output to support per-command help topics, and improves error messages. The changes are mechanical refactoring by the repository owner with no impact on core runtime behavior beyond CLI UX.

^{Macroscope would have approved this PR. Enable approvability here.}