feat(relay): tell clients what a working agent is doing

herdr already reports each pane's terminal title and the relay dropped it, so a
card could only ever say the harness name and the cwd -- the same two strings
for every pane in a worktree. A working claude sets that title to what it is
doing ("fix P0, draft the P1 plan"), which is the one field that distinguishes
three claudes in one directory.

Passing it through raw is worse than dropping it, which is why activity_title
exists rather than a plain `p.get(...)`. Measured on one real host with nine
agent panes: seven reported no title at all, and the two that reported one
reported the plain banner "Claude Code" -- the harness's own name, already in
the `agent` field right beside it, and worth less than the cwd a client drops in
order to show it. The banner is matched by prefix on an alphanumeric-only
flattening, so codex and opencode need no entry in a list, "✳ Claude Code"
matches too, and a title that merely mentions the harness ("porting the claude
reader") survives.

This is activity, not identity: it changes while the agent works and goes when
it stops. Nothing should treat it as a durable session name.