LintMarkdownTask + Gradle plugin functional tests

[roxspring]

Implement a Gradle task that reports Markdown style violations without modifying files, and add functional tests for the Gradle plugin using GradleRunner.

LintMarkdownTask scope

• @CacheableTask abstract class LintMarkdownTask : DefaultTask()
• @InputDirectory abstract val sourceDir: DirectoryProperty
• Configurable rules (each individually togglable):
  • Heading level skips (issue Lint rule: heading level skips #18)
  • Required front matter fields (issue Lint rule: required front matter fields #19)
  • Maximum line length (issue Lint rule: maximum line length #20)
  • One sentence per line (issue Lint rule: maximum line length #21)
• Violations reported as Gradle warnings with file + line number
• Task fails (throws GradleException) if any violations are found and failOnViolation = true (default)

Functional tests scope

• Use GradleRunner (Gradle TestKit) to exercise ProcessMarkdownTask, MergeMarkdownTask, and LintMarkdownTask against real project fixtures
• Tests live in gradle-plugin/src/test/kotlin/
• At minimum: one happy-path test and one failure-mode test per task

Acceptance criteria

• LintMarkdownTask reports violations with actionable messages
• Task fails build when violations found and failOnViolation = true
• At least one GradleRunner functional test passes for each task