feat: full-height right agent panel with per-agent last activity

The /agents-panel now fills the whole right column (40% width, 100% height,
top-right anchor). Under each running subagent it shows the agent's most
recent activity read from the tail of its session file: the last tool call
with its query (e.g. web_search: "...") or the last assistant text. Reads are
cached by file size so the 1.5s refresh stays cheap.

CHANGELOG.md

@@ -47,6 +47,10 @@
   panel with the live agent tree: non-capturing (typing keeps working),
   auto-refreshed every 1.5 s, hidden on terminals narrower than 100 columns,
   and closed automatically on shutdown.
+- The right-side panel now fills the whole right column (width 40%, full
+  height) and shows each running subagent's most recent activity under its
+  row — the last tool call with its query or the last assistant text — read
+  from the tail of the agent's session file (cached by file size).
 
 ## 0.1.1 — 2026-07-31