fix: make lint results independent of how the target is named

[nozaq]

Directory arguments were passed through almost verbatim, so whether a
target was named relatively or absolutely leaked past the reported paths
into rule verdicts and internal path resolution.

• id-dir-mismatch derived the containing directory's name from the
  reported path, which has no directory component when the target is
  named "." — the default. Linting a Feature or Template from inside it
  therefore always reported the id as mismatched. The name now comes
  from the directory's own location, handed to rules as linter.Dir.
• Compose resolution assumed the lint root was absolute, which only held
  when the target was named that way; a Dockerfile outside the
  configuration directory could otherwise be displayed as if inside it.
• The github format emitted absolute paths, which GitHub cannot place on
  a diff. Paths inside the working directory are now reported relative
  to it, with "/" separators.
• Arguments naming the same directory twice are linted once.
• Errors name the path once, cleaned, and say what to pass when given a
  file rather than a directory.

Co-Authored-By: Claude Opus 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01KsXG8rvSVXzUGYMe4AR5mi