shared terminal: align row icons via a shared grid, not offsets

Per mara's screenshot report (agent-terminal icons not aligned in the
first column) — and her explicit follow-up steer on the first version of
this fix: 'dont do it by offsets at all, it should be part of the layout
that they align.'

Root cause traced first, not guessed: .row-glyph relied on inheriting
text-indent: -1.4em from .live .row to paint its glyph in the reserved
prefix slot; details.row > summary separately reset text-indent: 0 for
its own hanging-caret needs, which also zeroed the inherited value for
the icon nested inside it (indent inherits by computed value, not by
rule). Two independently-computed offsets that had to agree by hand,
and silently didn't.

Replaced the whole offset scheme with a real structural fix: every row
is a 2-column CSS grid (icon column, width from one shared
--row-icon-col custom property, then content column), and a details
row grids its own <summary> with the exact same grid-template-columns
value instead of griding itself (so its <pre> body still stacks full
width below, not squeezed into column 2). Icon and content are placed
by explicit grid-column, not auto-placement inference, so an icon-less
row's lone child still lands in the content column. .row-content is a
new wrapper class (Row.tsx, terminal.js's row()/mutableRow()/
placeholder()) giving that content an element the grid can target by
class - a DocumentFragment (what linkify() returns) doesn't persist as
a node once appended, so without an explicit wrapper there was nothing
for the grid to place.

Caught and fixed a second real bug while building this for real (not
just reasoning about the CSS): the first draft's details.row { display:
block } had lower specificity than .live .row's display: grid and never
actually applied, squeezing <summary> into the row's own 1.4em icon
column and wrapping its text one character per line. Needed
.live details.row to out-specify it.

Verified with a headless-chromium render of all 6 row shapes (flat
icon / flat icon-less / flat markdown-body / details icon / details
icon-less / a long-wrapping flat row) - all align and wrap correctly.
Also verified via the earlier Range.getBoundingClientRect() measurement
(glyph paint position, not just the element's own box): diff 0.00 for
both this and the previous fix, but only this one is structural rather
than two numbers that happen to still agree today.

frontend/packages/agent/src/components/Row.tsx

@@ -66,7 +66,11 @@ export function Row({ row }: { row: TermRow }) {
     return (
       <div className={`row ${cssClass}`}>
         {icon}
-        {linkifyToNodes(row.summary)}
+        {/* Explicit wrapper, not bare children — the row's grid places
+            `.row-content` in its content column by class, not by
+            auto-placement inference (an icon-less row's lone child would
+            otherwise auto-place into the icon column instead). */}
+        <span className="row-content">{linkifyToNodes(row.summary)}</span>
       </div>
     );
   }