Revert "style: OpenCode Todo-style right agent panel (bg fill, checklist, pinned footer)"

This reverts commit e0f08097ec.

CHANGELOG.md

@@ -2,16 +2,6 @@
 
 ## Unreleased
 
-- Restyled the persistent right-side agent panel (`/agents-panel`) to match
-  OpenCode's Todo/plan sidebar: a full-height background column (no box border,
-  panel background painted across the whole overlay width) with a bold `▼ agents`
-  title, a flat OpenCode-style checklist (`[•]` in-progress in warning color,
-  `[ ]` pending and `[✓]` completed in muted, `[✖]` failed in error), live
-  per-agent activity lines under running agents, and the footer hint pinned to
-  the bottom of the column. The panel now sits flush against the right edge of
-  the terminal and stops two rows above the bottom so the editor and status bar
-  stay visible.
-
 - Fixed the research loop and agent tree getting permanently stuck after
   `research_control stop`: `start` now restarts a stopped/completed run in the
   same session (`AgentTree.restart` revives the run, archives old branches, and