warpgate-ssh: remote Linux/macOS terminals through the relay; oregon launched remotely

The dispatcher moves to an LXC without agent CLIs (design 9), so every terminal is
reached through Warpgate. The relay call (agent-access.py run --target <t>) lives in
one helper; the target comes from the terminal config.

os linux/darwin (PosixWarpgateTransport): each primitive is one bash -c call. Files
go up as base64 (chunked above 60k), start_run stages prompt.md + run.sh and launches
in a single round trip; the runner is detached with setsid nohup (pid == pgid == sid,
$! kept in launch.pid); kill signals the process group and session (TERM, then KILL),
skipped once exit.txt exists. macOS detaches with perl POSIX::setsid.

os windows keeps the win10-ps.sh / win10-put.sh helper scripts (target win10) and
schtasks, now through the shared call() helper.

oregon is now warpgate-ssh (target oregon, run_root ~/.gwent-board/agentd-runs);
the local transport stays for co-located setups and tests.

config/schema.json

@@ -48,7 +48,7 @@
       "properties": {
         "transport": {"enum": ["local", "warpgate-ssh"]},
         "target": {"type": "string"},
-        "os": {"enum": ["linux", "windows"]},
+        "os": {"enum": ["linux", "darwin", "windows"]},
         "launch": {"enum": ["subprocess", "nohup", "schtasks"]},
         "repo": {"type": "string"},
         "plane_user": {"type": "string"},