refactor(completion): name path arguments explicitly and share firstArg

The six commands that take a file or directory argument set their
completer directly instead of a Use-line parser wrapping every
command, the first-argument-only wrappers become one firstArg helper,
and only the bash completion help keeps a custom text.

Signed-off-by: bircni <bircni@icloud.com>

internal/app/action_jobs.go

@@ -38,7 +38,7 @@ $ tea run jobs 34 --json id,conclusion --jq '.[] | select(.conclusion == "failur
 		},
 	}
 	pageFlags(cmd, &limit, &page)
-	cmd.ValidArgsFunction = a.completeRunArg()
+	cmd.ValidArgsFunction = firstArg(a.completeRuns())
 	return cmd
 }