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>

internal/cli/interactive.go

@@ -73,6 +73,10 @@ func editInteractive(st *store.Store, name string, out, errOut io.Writer) int {
 // runInit creates/opens the DB and adds the first account via the TUI form,
 // seeding a default audit retention if unset.
 func runInit(args []string, out, errOut io.Writer) int {
+	if len(args) > 0 && helpRequested(args[0]) {
+		printCmdUsage(out, "init")
+		return 0
+	}
 	st, err := openStore()
 	if err != nil {
 		fmt.Fprintf(errOut, "emcli: %v\n", err)