Add hierarchical configuration for pony-lint

[SeanTAllen]

pony-lint now supports .pony-lint.json files in subdirectories, not just at the project root. A subdirectory config overrides the root config for all files in that subtree, using the same JSON format and proximity-first precedence.

New types: ConfigResolver resolves per-directory rule status by merging the config hierarchy. LintConfig.merge() layers child overrides on parent configs with category-cleaning semantics.

Config discovery piggybacks on the existing file-discovery walk alongside .gitignore/.ignore loading. Intermediate configs between the hierarchy root and walk targets are pre-loaded, and explicit file targets load their directory's config directly.

Malformed subdirectory configs produce lint/config-error diagnostics and fall through to the parent config.

Design: #5028

Things to note

1. Uncached RuleRegistry per file in text phase — _registry_for() constructs a new RuleRegistry for every file in an overridden directory. The config is cached but the registry is not. Negligible for a CLI tool; easy to add a registry cache later if needed.

2. Walk-discovered configs ignore empty root_dir sentinel — _FileCollector._load_config runs unconditionally during the walk regardless of whether hierarchical config is enabled (empty root_dir). No practical impact since main.pony always passes a non-empty root_dir.