feat(tui): include session name in the terminal titlebar (#43188)

The terminal/console titlebar was composed from status marker + model +
cwd only; the session's (auto-)title never appeared, even though the TUI
already knows it.

Change the format to `<marker> <session name> · <model> · <cwd>`, with the
session name and cwd each omitted when absent so single-segment titles stay
clean. The current session's live title is pulled from the existing
session.active_list poll (which already carries each session's current flag
and title), so there's no extra round-trip; UiState gains a sessionTitle
field updated only when it actually changes, preserving the existing
idle-flicker guard.

Extract the join logic into a pure composeTabTitle() helper in domain/paths
and cover its edge cases (name omitted, cwd omitted, whitespace-only name,
marker-only fallback, truncation, boundary length) in paths.test.ts.

ui-tui/src/domain/paths.ts

@@ -14,3 +14,27 @@ export const fmtCwdBranch = (cwd: string, branch: null | string, max = 40) => {
 
   return `${shortCwd(cwd, Math.max(8, max - tag.length))}${tag}`
 }
+
+/**
+ * Compose the terminal titlebar string:
+ *   `<marker> <session name> · <model> · <cwd>`
+ *
+ * The session name and cwd are each omitted when empty, and a long session
+ * name is truncated. The marker is always glued to the first present segment
+ * with a plain space (not a ` · ` separator). When no model is known yet the
+ * caller should fall back to a plain brand string instead of calling this.
+ */
+export const composeTabTitle = (
+  marker: string,
+  sessionName: string,
+  model: string,
+  cwd: string,
+  maxName = 28
+): string => {
+  const name = sessionName.trim()
+  const shortName = name.length > maxName ? `${name.slice(0, maxName - 1)}…` : name
+
+  const segments = [shortName, model, cwd].filter(Boolean)
+
+  return segments.length ? `${marker} ${segments.join(' · ')}` : marker
+}