feat(web): show and drive the terminals, not just the agents

The relay lists the panes with no agent in them; this renders them. They are a
Terminals section in the same list, with a hollow dot rather than a fourth shade
of grey competing with Done and Idle -- a terminal has no status to colour,
herdr reports agent_status "unknown" for every one of them.

The card's identity line is the pane id, not the harness name an agent card
carries. Measured on a real host: 20 shell panes sharing only 12 distinct cwd
basenames, so eight of them are indistinguishable from a sibling by directory
alone. The id always separates them, and the process name arrives on open.

Opening one differs from opening an agent pane in three ways, each tested rather
than asserted from the source:

- canLoadMore is true. Agent panes are on the alternate screen and report
  scrollback 0 without exception; these report up to 693, and the read costs
  10ms against a multi-second harvest. A ring is read with `recent`, a ringless
  pane with `visible` -- getting that backwards is what makes the relay pay.
- The History chip is hidden. There is no transcript; the relay would answer
  no-session.
- The opening read carries `process: true`, ONCE. It is one extra CLI call on
  the relay, which is one SSH round trip for a remote host, so the 3s mirror
  tick must not repeat it -- there is a test that fails if it does. When it
  arrives the title swaps the pane id for what is running: "…-dev · zsh".

Typing goes out as one `respond` rather than send_text + Enter, so the relay
audits it as respond_shell -- the line that says a command was run rather than
text typed at an agent. The agent path is untouched, and a test drives both to
prove the terminal branch did not swallow it.

With HERDR_SHELL_PANES off the `panes` key is absent and the page renders
exactly as before; that is a test too, since it is the state every existing
deployment is in.

Verified against the real relay and the real herdr, in chromium: 10 agents and
20 terminals listed, drilled into the focused workspace, opened wE:p2, and got
98 lines of its actual scrollback in colour under the title "herdr-remote-dev ·
zsh" with the History chip gone.