fix(tui): add missing alt+up and alt+down block cursor navigation bindings

Add alt+up and alt+down key bindings to the TUI app for block cursor navigation
as specified in the TUI specification. These bindings allow users to navigate
between conversation blocks using keyboard shortcuts.

- Add alt+up binding to move cursor to previous block
- Add alt+down binding to move cursor to next block
- Implement action_move_cursor_up and action_move_cursor_down methods
- Track current block index with _current_block_index
- Update help panel to display the new bindings in Main Screen context

src/cleveragents/tui/app.py

@@ -93,6 +93,8 @@ if _TEXTUAL_AVAILABLE:
             ("ctrl+q", "quit", "Quit"),
             ("f1", "help", "Help"),
             ("ctrl+t", "cycle_preset", "Cycle Preset"),
+            ("alt+up", "move_cursor_up", "Previous Block"),
+            ("alt+down", "move_cursor_down", "Next Block"),
         ]
 
         def __init__(
@@ -105,6 +107,7 @@ if _TEXTUAL_AVAILABLE:
             self._command_router = command_router
             self._persona_state = persona_state
             self._session = SessionView(session_id="default", transcript=[])
+            self._current_block_index: int = -1
 
         def compose(self) -> Any:
             yield _Header(show_clock=True)
@@ -152,6 +155,15 @@ if _TEXTUAL_AVAILABLE:
             self._persona_state.cycle_preset(self._session.session_id)
             self._refresh_persona_bar()
 
+        def action_move_cursor_up(self) -> None:
+            """Move block cursor to the previous block in the conversation."""
+            if self._current_block_index > 0:
+                self._current_block_index -= 1
+
+        def action_move_cursor_down(self) -> None:
+            """Move block cursor to the next block in the conversation."""
+            self._current_block_index += 1
+
         def _refresh_persona_bar(self) -> None:
             persona = self._persona_state.active_persona(self._session.session_id)
             preset = self._persona_state.current_preset(self._session.session_id)

src/cleveragents/tui/widgets/help_panel_overlay.py

@@ -33,6 +33,8 @@ _CONTEXT_ITEMS: dict[str, tuple[tuple[str, str], ...]] = {
     "Main Screen": (
         ("enter", "Submit prompt"),
         ("ctrl+t", "Cycle to next argument preset"),
+        ("alt+up", "Move to previous block"),
+        ("alt+down", "Move to next block"),
         ("@", "Open Reference Picker overlay"),
         ("/", "Open Slash Command overlay"),
         ("! / $", "Activate shell mode"),