Implement instance titles spec (shell-driven titles, stable numbering, overflow)

[n8bar]

Summary

Tracking implementation of docs/specs/instance-titles.md:

• Wire xterm.js onTitleChange (OSC 0 / 2) through to the server and broadcast to all connected clients.
• Stop renumbering auto-named instances when an instance is closed; assign the lowest unused integer for new instances and let the visible numbering have gaps.
• Topbar #active-session-title (src/client/index.html, src/client/style.css:797) ellipsizes when the shell-provided title overflows the topbar slot, and exposes the full title via a native title tooltip. Sidebar entries gain the same tooltip behavior.
• Display precedence: shell-provided title when present and non-empty; otherwise the workspace default (Instance N).

See the spec for the full set of acceptance checks and manual verification steps.

Found during

0.1.1 dogfooding on 2026-05-23. Two related observations:

• CLI apps cannot change the terminal title at all in TermiWeb.
• Closing an instance renumbers the others, which makes the instance number useless as a mental identifier.

Release lane

Not yet assigned. Pending product decision on which release line this work belongs to.