Adds Markdown output for the check command.

Scope:

• add --format markdown to check;
• keep console output as default;
• keep JSON output working;
• emit readable Markdown for ok, empty and error outcomes;
• preserve exit codes 0, 1 and 2;
• redact secret-like repository values in Markdown output;
• add CLI tests for Markdown output and redaction.

Validation:

• ./scripts/check.sh passed with 34 tests;
• console output verified;
• JSON output verified;
• Markdown ok output verified;
• Markdown empty output returns exit 1;
• Markdown error output returns exit 2;
• secret-like path values are redacted;
• no raw secret-like literals in changed files.