fix(config): resolve markdownlint-cli2 configs like markdownlint-cli2 - #942

Description

Fixes #941.

rumdl reads .markdownlint-cli2.* and .markdownlint.* files on behalf of markdownlint-cli2, so I think it should resolve them the same way markdownlint-cli2 does. Two things differed:

1. A nested .markdownlint-cli2.* file replaced the config from parent directories, so rules it didn't mention fell back to rumdl's defaults.
2. A .markdownlint-cli2.* file took precedence over a .markdownlint.* file in the same directory. markdownlint-cli2 does the opposite (config: markdownlintConfig || markdownlintOptions?.config in lintFiles).

I based the resolution on markdownlint-cli2's source (createDirInfos, mergeOptions, lintFiles) and checked each case against markdownlint-cli2 v0.23.3:

• Nested .markdownlint-cli2.* configs merge over their parents. Like mergeOptions, a rule key in a nearer file replaces the inherited entry as a whole (no deep merge of rule options).
• A .markdownlint.* file is used as-is. It beats a .markdownlint-cli2.* config in the same directory, and a nested directory inherits it while no .markdownlint-cli2.* file in between sets config.
• Inheritance stops at the directory rumdl runs from, since markdownlint-cli2 stops at its working directory. For the LSP that's the document's workspace root.
• Explicit --config files stay standalone. The public SourcedConfig::load_discovered keeps its signature and behavior; the LSP uses a new crate-private load_discovered_within that takes the workspace root.

MARKDOWNLINT_CONFIG_FILES now lists .markdownlint.* before .markdownlint-cli2.* (and .jsonc before .json, matching markdownlint-cli2's order).

Known limitations:

• Inherited values are attributed to the nearest file in provenance output (e.g. rumdl config), although every file in the chain is listed in loaded_files.
• For a file outside the working directory, markdownlint-cli2 injects the working directory's config as a parent. rumdl doesn't inherit anything in that case.

Type of Change

• Bug fix (non-breaking change fixing an issue)
• New feature (non-breaking change adding functionality)
• Breaking change (fix or feature causing existing functionality to change)
• Documentation update

Testing

• Tests added/updated
• All tests passing (make test-dev)
• Manual testing performed

make test-dev has 14 failures locally, and the same tests fail on main for me: the code block tool tests need gofmt, prettier, ruff, and similar tools I don't have installed, and a_filtered_file_under_a_vendor_directory_still_counts fails too.

Checklist

• Code follows project style (make fmt && make lint)
• Conventional commit messages used (they generate the changelog)
• Documentation updated (if needed)

Clippy passes. lint-actions couldn't run locally because shellcheck isn't installed.