Fix --via/-J grammar and --force process-group kill in spec

Found both while translating the spec into code:

- `ssh -J a,b,c` alone isn't valid - ssh -J only carries jump hosts
  before the final hop; it still needs a positional destination. --via's
  last entry has to double as that target, which also makes --via
  mandatory (there's no other field naming "the host ssh connects to"),
  reversing the earlier "no" in the required column.
- A plain single-pid SIGKILL on `close --force` would leave the `ssh`
  child orphaned, since a killed process can't forward anything to it.
  Documents sending SIGKILL to the whole process group instead, relying
  on the supervisor's own setsid() call making it the group leader.

.gitignore

@@ -4,3 +4,4 @@
 
 target/
 *.lock
+!Cargo.lock