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/help_test.go

@@ -15,7 +15,7 @@ func TestMainHelpListsAllCommands(t *testing.T) {
 			t.Fatalf("%v: want exit 0, got %d\n%s", args, code, text)
 		}
 		for _, want := range []string{
-			"Usage", "list", "get", "search", "ack", "send",
+			"Usage", "list", "get", "search", "ack", "send", "folders",
 			"account", "whitelist", "config", "audit", "doctor", "version",
 			"EMCLI_KEY", "EMCLI_DB",
 		} {