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/mail/imap_integration_test.go

@@ -49,6 +49,25 @@ func TestSelectAndFetch(t *testing.T) {
 	t.Logf("inbox has %d messages, maxUID=%d", len(headers), maxUID)
 }
 
+func TestListFolders(t *testing.T) {
+	c, err := Dial(testCfg(t))
+	if err != nil {
+		t.Fatalf("Dial: %v", err)
+	}
+	defer c.Logout()
+
+	folders, err := c.ListFolders()
+	if err != nil {
+		t.Fatalf("ListFolders: %v", err)
+	}
+	if len(folders) == 0 {
+		t.Fatal("expected at least one folder")
+	}
+	if folders[0].Name != "INBOX" || !folders[0].Selectable {
+		t.Fatalf("expected selectable INBOX first, got %+v", folders[0])
+	}
+}
+
 func atoiEnv(t *testing.T, k string) int {
 	t.Helper()
 	n, err := strconv.Atoi(os.Getenv(k))