Three-pane subconscious debug screen with shared widgets

New layout for F3 screen:
- Top-left: agent list using ratatui List widget with ListState
- Middle-left: expandable agent state (persistent across runs)
- Bottom-left: memory store activity by provenance, walked keys
- Right: context tree from fork point, reusing SectionTree

Tab/Shift-Tab cycles focus clockwise between panes; focused pane
gets white border. Each pane handles its own input when focused.

Extracted user/widgets.rs:
- SectionTree (moved from mod.rs): expand/collapse tree for ContextSection
- pane_block_focused(): standard bordered block with focus indicator
- format_age()/format_ts_age(): shared duration formatting

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

src/user/mod.rs

@@ -8,6 +8,7 @@ mod context;
 mod subconscious;
 mod unconscious;
 mod thalamus;
+mod widgets;
 
 use anyhow::Result;
 use std::io::Write;