Show every agent tab's summary, not just the focused one

The tab bar's row-2 summary was painted only for the active tab. Add a
per-child summaryTextFor/summaryRawFor helper (active variants now
delegate to it), carry each tab's childID on its tabRect, and loop over
all visible tabs so each renders its own summary under its column.
Layout is unchanged (still 3 rows); narrow tabs clip as before.

Resolves the per-tab summary TODO item.

CHANGELOG.md

@@ -11,6 +11,8 @@ loosely follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
   to remove a shared project scratchpad.
 
 ### Changed
+- The tab bar now shows each visible agent tab's own summary instead
+  of only rendering the focused tab's summary.
 - Grid-mode `get_process_output` now returns whitespace-normalized
   text to avoid sending padded terminal rows and repeated blank lines
   over MCP.