feat(cli): folders command, search --all-folders, empty-search hint

Fixes from testing email search (docs/enhancements-2026-07-07.md):

- New `folders` agent command lists the account's mailboxes (name,
  delimiter, selectable), INBOX first, so agents can discover archived
  mail outside INBOX.
- `search --all-folders` sweeps every selectable mailbox; each hit
  carries a `folder` field, `skipped_folders` reports mailboxes the
  server refused, and --limit caps visible results across the sweep.
  The sweep deliberately skips EnsureFolderBaseline so a read-only
  search never mutates list --new state.
- Empty search results include a generic `data.hint` with next steps.
  The hint is a fixed constant per mode, so the invisibility invariant
  holds: absent and policy-filtered mail produce byte-identical
  envelopes (codified in TestSearchEmptyHintIndistinguishableFromFiltered).
- Skill and user docs: document `--text` full-text search as
  best-effort (server-dependent); recommend --subject-contains/--from.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/cli/run.go

@@ -165,6 +165,8 @@ func Run(args []string, out, errOut io.Writer) int {
 	switch normalizeVerb(cmd) {
 	case "list", "get", "search", "ack":
 		return runAgent(normalizeVerb(cmd), rest, role, out, errOut)
+	case "folders":
+		return runFolders(rest, role, out, errOut)
 	case "send":
 		return runSend(rest, role, out, errOut)
 	case "account":
@@ -185,6 +187,37 @@ func Run(args []string, out, errOut io.Writer) int {
 	}
 }
 
+// runFolders handles `folders --account <name>` — list the account's mailboxes.
+func runFolders(args []string, role store.Role, out, errOut io.Writer) int {
+	fs := flag.NewFlagSet("folders", flag.ContinueOnError)
+	fs.SetOutput(errOut)
+	usageFlags(fs, "folders", errOut)
+	account := fs.String("account", "", "account name")
+	if err := fs.Parse(args); err != nil {
+		if errors.Is(err, flag.ErrHelp) {
+			return 0
+		}
+		_ = Failure(CodeUsage, err.Error()).Write(out)
+		return 2
+	}
+	if *account == "" {
+		_ = Failure(CodeUsage, "--account is required").Write(out)
+		return 2
+	}
+	st, err := openStore(role)
+	if err != nil {
+		_ = Failure(CodeConfig, err.Error()).Write(out)
+		return 1
+	}
+	defer st.Close()
+	_, _ = st.PurgeAudit(time.Now())
+	d := newDepsLive(st, out)
+	if err := FoldersCmd(d, *account); err != nil {
+		return 1
+	}
+	return 0
+}
+
 // runAgent handles JSON-emitting commands. Errors are emitted as JSON envelopes.
 func runAgent(cmd string, args []string, role store.Role, out, errOut io.Writer) int {
 	fs := flag.NewFlagSet(cmd, flag.ContinueOnError)
@@ -203,6 +236,10 @@ func runAgent(cmd string, args []string, role store.Role, out, errOut io.Writer)
 	sinceDate := fs.String("since-date", "", "search: RFC3339 date lower bound")
 	beforeDate := fs.String("before-date", "", "search: RFC3339 date upper bound")
 	ackUIDs := fs.String("uid-list", "", "ack: comma-separated UIDs")
+	allFolders := new(bool)
+	if cmd == "search" {
+		allFolders = fs.Bool("all-folders", false, "search: sweep all mailboxes")
+	}
 	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
@@ -217,6 +254,18 @@ func runAgent(cmd string, args []string, role store.Role, out, errOut io.Writer)
 		_ = Failure(CodeUsage, "--account is required").Write(out)
 		return 2
 	}
+	if *allFolders {
+		folderSet := false
+		fs.Visit(func(f *flag.Flag) {
+			if f.Name == "folder" {
+				folderSet = true
+			}
+		})
+		if folderSet {
+			_ = Failure(CodeUsage, "--folder and --all-folders are mutually exclusive").Write(out)
+			return 2
+		}
+	}
 	st, err := openStore(role)
 	if err != nil {
 		_ = Failure(CodeConfig, err.Error()).Write(out)
@@ -251,7 +300,11 @@ func runAgent(cmd string, args []string, role store.Role, out, errOut io.Writer)
 				sc.Before = tm
 			}
 		}
-		if err := SearchCmd(d, *account, *folder, sc, *limit); err != nil {
+		if *allFolders {
+			if err := SearchAllCmd(d, *account, sc, *limit); err != nil {
+				return 1
+			}
+		} else if err := SearchCmd(d, *account, *folder, sc, *limit); err != nil {
 			return 1
 		}
 	case "ack":