Rename our "host" vocabulary to "agent", the term DWService uses - #5

dwshell called the machine you connect to a "host". DWService does not:
its own English strings shipped with the agent say "Agent name",
"Creating a new agent", "The agent {0} already exists" — 39 occurrences
of "agent" against one of "host", and that one is `proxyHost`, a network
proxy. The protocol modules are `agent`, `share` and `group`.

Speaking the service's language matters more now that agent management
is coming: a command group named after the thing it manages reads wrong
if the rest of the CLI calls that thing something else.

Not a breaking change: "host" was never a flag or a config key, only
variable names, help text and prose.

Three things keep the word, because there it means a network host and
renaming would be wrong or would falsify a record:

  - internal/client — the local machine's hostname, used to label a
    registered trusted device ("dwshell on <hostname>");
  - internal/session/socket.go — the relay host (u.Host);
  - docs/PROTOCOL.md — "Hosts and terminology" is about the service's
    own domains, and two further hits are captured terminal output
    (user@host:~#) and a file literally named "hosts";
  - docs/DESIGN.md keeps "conhost", the Windows console host, and the
    README keeps "repositories are hosted at".

parseUserHost becomes parseUserAndAgent rather than parseUserAgent,
which would read as the HTTP header.

Pure rename: 130 insertions against 130 deletions, no logic touched.
Verified beyond the suite by exercising the real paths against live
agents — list, -c with its exit code, and ls on a remote directory.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01MvidAFW9a2r4hTgHPW9ywG