design(remote): capability tiers and the host profile

[devsuitup]

A declared host offers different things depending on what runs on it. Today the
UI assumes one shape and the code assumes another, so a host that cannot do
something fails at the click instead of saying so.

Proposed ladder, each tier naming what it requires:

1. observe — transcripts only. Works anywhere ssh and a readable
   ~/.claude/projects exist. Shipped.
2. liveness — requires ~/.claude/sessions/<pid>.json. No multiplexer
   needed.
3. inject — requires a channel; see the injection issue.
4. attach — requires a multiplexer named in the descriptor.
5. launch — requires a multiplexer, or degrades to headless.

Scope: a host profile computed from the descriptors and a small probe, a tier
shown in the UI, and every affordance above a host's tier disabled with the
reason stated. No feature should be discovered to be missing by failing.