Add expand/collapse all, per-pane key legends

SectionTree:
- 'e': expand all nodes
- 'c': collapse all nodes
- Home/End already wired from previous commit

Key legend shown at bottom border of each focused pane:
- Tree panes: nav, expand/collapse, expand/collapse all, paging
- Agent list: select, tab
- History: scroll, paging

Legend only appears on the focused pane to avoid clutter.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/user/context.rs

@@ -11,7 +11,7 @@ use ratatui::{
 };
 
 use super::{App, ScreenView, screen_legend};
-use super::widgets::{SectionTree, pane_block, render_scrollable};
+use super::widgets::{SectionTree, pane_block, render_scrollable, tree_legend};
 
 pub(crate) struct ConsciousScreen {
     agent: std::sync::Arc<tokio::sync::Mutex<crate::agent::Agent>>,
@@ -94,7 +94,8 @@ impl ScreenView for ConsciousScreen {
         lines.push(Line::raw(format!("  Active tools: {}", app.active_tools.lock().unwrap().len())));
 
         let block = pane_block("context")
-            .title_top(Line::from(screen_legend()).left_aligned());
+            .title_top(Line::from(screen_legend()).left_aligned())
+            .title_bottom(tree_legend());
 
         render_scrollable(frame, area, lines, block, self.tree.scroll);
     }