feat(cli): add help for all commands

emcli had only raw flag usage and no command listing; `--help` on agent commands
even emitted a JSON error envelope and exited 2. Add real help:

- Top-level `emcli` / `help` / `-h` / `--help` prints a grouped command catalogue
  (agent vs admin) with one-line summaries and the EMCLI_KEY/EMCLI_DB env vars.
- `emcli help <command>` prints that command's synopsis + summary.
- `emcli <command> --help` prints synopsis + summary + flags and exits 0. Agent
  commands keep stdout JSON-free (usage goes to stderr); admin commands print to
  stdout. Help works without EMCLI_KEY (no DB access).
- help.go holds the command catalogue; flag.ErrHelp is handled as success, and
  admin handlers short-circuit help before opening the store.

Unknown commands still error (exit 2). Full suite passes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

USER-MANUAL.md

@@ -524,6 +524,10 @@ running non-interactively.
 ## 12. Command cheat sheet
 
 ```
+# Help
+emcli                  # or: emcli help / emcli --help — list all commands
+emcli <command> --help # usage and flags for one command
+
 # Admin
 emcli init                                            # create DB + add first account (form)
 emcli account add [flags | none for form]            # add an account