Keep file/line in the github reporter's log output

[wilfison]

Closes #495

Problem

The github reporter emits GitHub Actions workflow commands like:

::error file=app/views/x.haml,line=12::Message

GitHub moves the file/line of a workflow command into the annotation shown in the "Files changed" tab, so they disappear from the plain Actions log — leaving only the bare message with no location. This makes the log much less useful than the default reporter, which is the trade-off reported in #495.

Change

Enhance Reporter::GithubReporter so the location is preserved in both places:

• Location stays in the log — the command's message is now prefixed with
  filename:line LinterName:, so the log line reads e.g.
  app/views/x.haml:12 RuboCop: Style/StringLiterals: ... even after GitHub strips the
  file/line into the annotation.
• Annotations get a title — title=haml-lint <LinterName> (falling back to haml-lint
  when a lint has no linter), so annotations are labeled and distinguishable from other tools'.
• Correct escaping — message values escape %, \n, \r; property values (file,
  title, ...) additionally escape the : and , that delimit the command.

Before

::error file=app/views/x.haml,line=12::Avoid defining `class` in attributes hash...

After

::error file=app/views/x.haml,line=12,title=haml-lint ClassAttributeWithStaticValue::app/views/x.haml:12 ClassAttributeWithStaticValue: Avoid defining `class` in attributes hash...

The annotation in the Files tab still works exactly as before; the difference is that the log
now keeps the file, line, and linter name.

Tests

• Updated the file/line assertions to target the file=/line= command properties.
• Added coverage for the annotation title, the location/linter prefix in the message (with and
  without an associated linter), and escaping of delimiter characters.
• bundle exec rspec spec/haml_lint/reporter/github_reporter_spec.rb — 18 examples, 0 failures.
• bundle exec rubocop lib/haml_lint/reporter/github_reporter.rb — no offenses.