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.
d33965396e