Show idle state in the top tab bar

Each agent tab now prefixes its label with the same one-rune idle
indicator the sidebar uses (✕ error, ? permission, ◐ thinking, ○ idle,
● working), so the state of every open agent is visible without
opening or focusing each tab. Tab redraws now fire on idle-state
changes in addition to sidebar redraws.

CHANGELOG.md

@@ -6,6 +6,12 @@ loosely follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+### Added
+- The top tab bar now prefixes each agent tab's label with its
+  idle-state glyph (✕ error, ? permission, ◐ thinking, ○ idle, ●
+  working), matching the sidebar's vocabulary so the state of every
+  open agent is visible without opening or focusing each tab.
+
 ### Changed
 - Built-in agent presets (`claude`, `codex`, `opencode`) now live in
   memory and user preset files merge over them by name instead of