Room polish: typing indicator, sending spinner, copy

[udaychandra]

Field-feedback fixes from the first live engagement session:

• Typing indicator in the message list — iMessage-style pulsing dots under the agent's avatar as the last row while its chat turn runs (replaces the composer status line, which is removed entirely along with the "No agent in this room" whisper). Respects prefers-reduced-motion; the room autoscrolls when the indicator appears while pinned.
• It appears at launch, not at first tool call. The presence store now seeds a working entry from agent_session_started (using the run_role sail ≥0.28.1 stamps on it; older servers still light the headline pill). This is where the ~10s dead air between sending and the working pill came from — the launch event was being dropped on the floor.
• Sending spinner — pending messages show a small spinner beside "Sending…" with proper spacing.
• Copy sweep — em-dashes removed from the Add-agent dialog, toasts, and room whispers; the two flagged hints now read "It can draft specs and work in the workspace." and "The agent joins immediately. There is no rollback point, so undo any damage by hand."

typecheck + 540 tests green (new: presence-at-launch matrix incl. per-run stop isolation and the pre-0.28.1 roleless fallback).