Add modal editing to the agent TUI. (#2)

Adds a status line indicating which mode the user is in.
Adds a "normal" mode with keyboard shortcuts (including a chorded shortcut 'gg').
Adds a command mode with several basic commands that can be entered into an overlay.

Chores:
- Cleans up design/claude/plan.md to avoid confusing claude.
- Adds some TODOs based on claude feedback.`

Reviewed-on: #2
Co-authored-by: Drew Galbraith <drew@tiramisu.one>
Co-committed-by: Drew Galbraith <drew@tiramisu.one>

IDEAS.md

@@ -42,6 +42,9 @@ Notes based on ideas I've had.
 - JSONL `SessionWriter` with `Event` structure
 - Parent IDs, timestamps, token usage per event
 - Predictable file location with session IDs
+- Ability to resume prior session.
 - **Done when:** Session files are coherent, parseable, with token counts per turn
 
-
+## Which-key like help
+- Show command chording in normal mode.
+- Help window slides up from bottom of screen.