Marquee long sidebar names; truncate with ellipsis otherwise

Sidebar rows that overflow the rail width used to spill characters
into the main viewport. They now truncate with a trailing "…"
when unfocused (or when the focused name still fits). The focused
row whose name overflows runs a pause-scroll-pause marquee: 1 s
hold on the head, ~150 ms per cell scroll, 1 s hold on the tail,
snap back. The row's geometry never moves while it animates, so
nothing below shifts.

A dedicated 150 ms goroutine flips sidebarDirty only while a row
is actively animating; the chrome ticker does the actual repaint.
Idle is a single cheap wakeup. focus / spawn / exit / restart all
reset the marquee state so the new focused row starts from frame
zero. When the row's budget is tight, the trailing timer
indicator drops before the name ellipses since the name is the
only identifier the row carries.

clampVisible() is a defensive net inside write(): even if a row's
decoration size were mis-computed, it will not spill past the
sidebar band into the PTY area.

CHANGELOG.md

@@ -13,6 +13,16 @@ loosely follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
   the tab would lose its highlight as soon as you stepped into a
   child agent, even though you were still within that thread.
 
+### Changed
+- Sidebar rows (Processes, Agent Tree, Scratchpads) now truncate
+  overflowing names with a trailing `…` instead of spilling into
+  the main viewport. The focused row marquees its name when it
+  overflows — 1 s hold on the head, ~150 ms per cell scroll until
+  the tail is visible, 1 s hold on the tail, snap back. Row
+  position never moves while the marquee animates. When budget is
+  tight, the trailing timer indicator drops before the name
+  ellipses, since the name is the only identifier the row carries.
+
 ## [0.0.2] - 2026-05-15
 
 ### Added