Add a diagnostics/doctor mode for integration troubleshooting (CLI flag + sbt task)

[miroslavpojer]

Feature Description

Provide a “doctor” mode that prints resolved configuration and common failure checks for both CLI usage and sbt plugin usage (e.g. --diagnostics and jmfDoctor).

Problem / Opportunity

• When coverage output is wrong, users don’t know if the rewriter ran, rules matched, paths were correct, or JaCoCo used filtered classes.
• Debugging integration issues currently requires reading plugin internals or enabling ad-hoc logging.

Acceptance Criteria

1. CLI diagnostics prints: resolved --in, --out, --rules, number of class files scanned, number of methods marked, and (optional) a few sample matches.
2. sbt task prints: resolved rules file path, input/output class directories per module, whether enabled, and where reports will be generated.
3. Diagnostics output is stable and copy-pasteable into bug reports.
4. Diagnostics mode does not modify classes unless explicitly requested (safe by default).

Proposed Solution

• Add a structured, human-readable report (and optionally JSON) for troubleshooting.
• Reuse the same underlying reporting logic in CLI and sbt plugin when possible.

Dependencies / Related

Complements the existing dry-run functionality; not the same as #6/#16/#17.

Additional Context

This should reduce support load and speed up adoption in CI pipelines.