fix(r): correct shell quote escaping in lintr command

[sims1253]

Bug: The R language plugin's lintr command had broken shell quote escaping that prevented it from executing properly.

Root Cause:

1. Quote collision: The outer Python string used single quotes, but the shell command also used single quotes for the R expression, causing the shell to receive malformed commands.
2. Over-engineering: The cat(paste(capture.output(...), collapse="\n")) wrapper was unnecessary — lintr::lint_dir() already prints to stdout by default.

The Fix:

     # Before (broken):
     'cmd': (
         'Rscript -e \'cat(paste(capture.output('
         'lintr::lint_dir(".", show_notifications=FALSE)'
         '), collapse="\\n"))\''
     ),

     # After (fixed):
     "cmd": (
         "Rscript -e \"lintr::lint_dir('.')\""
     ),

Changes:
• Switched to double quotes for the Python string, allowing single quotes inside for the R string
• Removed unnecessary cat/paste/capture.output wrapper
• Simplified to rely on lintr's default stdout behavior
• Output remains GNU-style formatted lint messages (compatible with fmt: "gnu")