fix(tui): stop slash dropdown from chopping last char of /goal (#31311)

Two independent bugs caused the slash-command autocomplete to render
`/goal` as `/goa` (and `/gquota` as `/gquot` for that matter) in the TUI:

1. `tui_gateway/server.py` was forwarding `c.display` from
   prompt_toolkit's `Completion` straight into the JSON-RPC payload.
   prompt_toolkit normalizes `display=` into `FormattedText` (a `list`
   subclass), so the wire format became `[["", "/goal"]]` instead of
   the `string` that `CompletionItem.display` in the TUI declares.
   `meta` already went through `to_plain_text` — `display` did not.

2. The dropdown row in `appOverlays.tsx` used `flexDirection="row"`
   with the display `<Text>` and the (very long) meta `<Text>` as
   siblings. When the meta overflows the row width, Ink/Yoga shrinks
   the *first* column by one cell, lopping the trailing character off
   the command name. `/goal` triggers it reliably because its meta
   string is the longest of any built-in command (description +
   embedded `[text | pause | resume | clear | status]` usage hint).
   Wrapping the display column in `<Box flexShrink={0}>` keeps it at
   its natural width and lets the meta wrap or truncate instead.

ui-tui/src/components/appOverlays.tsx

@@ -187,10 +187,15 @@ export function FloatingOverlays({
                   key={`${start + i}:${item.text}:${item.display}:${item.meta ?? ''}`}
                   width="100%"
                 >
-                  <Text bold color={theme.color.label}>
-                    {' '}
-                    {item.display}
-                  </Text>
+                  {/* flexShrink=0 — when meta overflows the row, Ink/Yoga
+                      otherwise shaves the last char off the display column
+                      (e.g. /goal renders as /goa). */}
+                  <Box flexShrink={0}>
+                    <Text bold color={theme.color.label}>
+                      {' '}
+                      {item.display}
+                    </Text>
+                  </Box>
                   {item.meta ? (
                     <Text
                       backgroundColor={active ? theme.color.completionMetaCurrentBg : theme.color.completionMetaBg}