[ajianaz]

What

• Config::validate (so config load and cora config validate) now rejects an invalid regex, an empty or duplicate id, an id shadowing a built-in rule, an empty message, an empty languages list and unknown languages values, naming the rule.
• languages accepts language names (rust) as well as extensions (rs); languages and exclude are optional.
• cora config validate --dry-run [--path DIR] runs each custom rule over the project and prints how many lines it matched (0 matches is flagged).
• docs/configuration.md shows the real schema (rules_engine.custom, the free-text rules: list, required fields); the 0.19.0 changelog line says custom.

Why

A mistyped pattern or language made a rule silently inactive while config validate said valid. Closes #684, closes #678.

Testing

• Unit tests for validation, language normalisation and optional YAML fields.
• 4 end-to-end tests in tests/custom_rules.rs (invalid regex fails validation, docs example loads and fires, unknown language, dry run flags a silent rule).
• fmt, clippy -D warnings and the full suite pass.