Expand CLI help into a discoverable command and option reference

[dapi]

Problem

code-converge --help currently prints only:

usage: code-converge [flags] [config]

The command gives no way to discover:

• available commands (config, update);
• supported global flags;
• configuration entry points and the primary documentation;
• command-specific syntax.

Additionally, code-converge config --help and code-converge update --help currently fail with exit code 2, rather than presenting successful help.

The minimal root-help output was intentionally introduced by #20, but it is insufficient for interactive operators and makes normal CLI discovery depend on reading the README.

Desired outcome

Provide concise, stable, human-oriented help without loading configuration, starting a workflow, or checking for updates.

Acceptance criteria

• code-converge -h and code-converge --help exit 0 and show:
  • root usage;
  • command synopsis for config and update;
  • a concise list of global options, grouped where helpful;
  • a pointer to the full README/configuration reference.
• code-converge config --help exits 0 and explains its purpose and valid invocation.
• code-converge update --help exits 0 and documents --yes / -y.
• Help never loads project/user configuration, creates diagnostic session logs, starts Codex, or invokes self-update.
• The help text and exit semantics are covered by focused tests and documented in the public CLI contract.
• Machine-readable workflow output remains unaffected.

Scope notes

This changes the public CLI/stdout contract, so it should use Feature Flow rather than Small Change. The implementation should decide whether the detailed option list is generated from the flag definitions to prevent drift.

[dapi]

Feature package: memory-bank/features/FT-036/brief.md; solution and execution artifacts are linked from that package. Delivery PR: #37.