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/run.go

@@ -1,6 +1,7 @@
 package cli
 
 import (
+	"errors"
 	"flag"
 	"fmt"
 	"io"
@@ -70,8 +71,12 @@ func newDepsLive(st *store.Store, out io.Writer) Deps {
 func runDoctor(args []string, out, errOut io.Writer) int {
 	fs := flag.NewFlagSet("doctor", flag.ContinueOnError)
 	fs.SetOutput(errOut)
+	usageFlags(fs, "doctor", errOut)
 	account := fs.String("account", "", "check only this account")
 	if err := fs.Parse(args); err != nil {
+		if errors.Is(err, flag.ErrHelp) {
+			return 0
+		}
 		return 2
 	}
 	st, err := openStore()
@@ -89,9 +94,14 @@ func runDoctor(args []string, out, errOut io.Writer) int {
 
 // Run routes a command line and returns an exit code.
 func Run(args []string, out, errOut io.Writer) int {
-	if len(args) == 0 {
-		fmt.Fprintln(errOut, "emcli: no command given")
-		return 2
+	if len(args) == 0 || helpRequested(args[0]) {
+		// `emcli`, `emcli help`, `emcli -h`, `emcli --help`, and `emcli help <cmd>`.
+		if len(args) >= 2 {
+			printCmdUsage(out, args[1])
+		} else {
+			printMainHelp(out)
+		}
+		return 0
 	}
 	cmd, rest := args[0], args[1:]
 	switch cmd {
@@ -121,6 +131,7 @@ func Run(args []string, out, errOut io.Writer) int {
 func runAgent(cmd string, args []string, out, errOut io.Writer) int {
 	fs := flag.NewFlagSet(cmd, flag.ContinueOnError)
 	fs.SetOutput(errOut)
+	usageFlags(fs, cmd, errOut)
 	account := fs.String("account", "", "account name")
 	folder := fs.String("folder", "INBOX", "folder/mailbox")
 	onlyNew := fs.Bool("new", false, "only new (unacked) messages")
@@ -135,6 +146,9 @@ func runAgent(cmd string, args []string, out, errOut io.Writer) int {
 	beforeDate := fs.String("before-date", "", "search: RFC3339 date upper bound")
 	ackUIDs := fs.String("uid-list", "", "ack: comma-separated UIDs")
 	if err := fs.Parse(args); err != nil {
+		if errors.Is(err, flag.ErrHelp) {
+			return 0 // usage already printed to stderr; help isn't an error
+		}
 		_ = Failure(CodeUsage, err.Error()).Write(out)
 		return 2
 	}
@@ -213,6 +227,7 @@ func (s *stringSlice) Set(v string) error {
 func runSend(args []string, out, errOut io.Writer) int {
 	fs := flag.NewFlagSet("send", flag.ContinueOnError)
 	fs.SetOutput(errOut)
+	usageFlags(fs, "send", errOut)
 	account := fs.String("account", "", "account name")
 	var to, cc, bcc, attach stringSlice
 	fs.Var(&to, "to", "recipient (repeatable / comma-separated)")
@@ -224,6 +239,9 @@ func runSend(args []string, out, errOut io.Writer) int {
 	replyTo := fs.Uint("reply-to", 0, "source UID to reply to (threading)")
 	folder := fs.String("folder", "INBOX", "folder of the reply source")
 	if err := fs.Parse(args); err != nil {
+		if errors.Is(err, flag.ErrHelp) {
+			return 0
+		}
 		_ = Failure(CodeUsage, err.Error()).Write(out)
 		return 2
 	}