feat(tui): hidden mode folds a turn's assistant steps into one message

The Ctrl+O hidden phase keeps one Assistant header per turn: the first
step with visible text/reasoning owns it, later steps render as
headerless continuations, and bodiless (tool-only) steps render
nothing. Leaving hidden restores per-step headers. Pure TUI
presentation; the session log is unchanged.

packages/ui/tui/tests/snapshots/tool-cards-hidden-folded.expected.txt

@@ -0,0 +1,46 @@
+terminal 100x40 buffer=normal length=40 base=0 viewport=0
+lifecycle started=1 stopped=0 progress=inactive
+title "DSH snapshot"
+cursor hidden column=7 viewportRow=20 bufferRow=20
+buffer
+0| " DEEPSEEK HARNESS"
+  style 1-8 fg=bright-magenta bold
+  style 10-16 bold
+1| " Snapshot agent ready."
+  style 1-21 dim
+2| " main-session"
+  style 1-12 dim
+3| <blank>
+4| "Assistant                                                                                           "
+  style 0-8 fg=bright-magenta bold underline
+5| "Inspecting the renderer first.                                                                      "
+6| "Model wait 0.0s                                                                                     "
+  style 0-14 dim
+7| <blank>
+8| "You                                                                                                 "
+  style 0-2 fg=bright-magenta bold underline
+9| "Refactor the renderer.                                                                              "
+10| "Model wait 0.0s · Completed 2026-07-29 22:23:17                                                     "
+  style 0-46 dim
+11| <blank>
+12| "The renderer is sound; no refactor needed.                                                          "
+13| "Model wait 0.0s · Completed 2026-07-29 22:23:17                                                     "
+  style 0-46 dim
+14| <blank>
+15| "Tool and context cards expanded.                                                                    "
+  style 0-31 dim
+16| <blank>
+17| "Tool cards hidden.                                                                                  "
+  style 0-17 dim
+18| <blank>
+19| "/workspace/project (tui-staging)  deepseek-v4-flash  ↑0 ↓0  0% context"
+  style 0-17 fg=bright-magenta bold
+  style 18-31 dim
+  style 34-50 dim
+  style 53-57 dim
+  style 60-69 dim
+20| " dsh >                                                                                              "
+  style 1-3 fg=bright-magenta bold
+  style 5-6 dim
+  style 7-7 inverse
+21-39| <blank>