fix(plugins): credit oxlint config packages referenced via the extends array

[BartWaardenburg]

In crates/core/src/plugins/oxlint.rs, OxlintPlugin::resolve_config now parses the top-level "extends" array from oxlint config files. Package-style entries (e.g. @nkzw/oxlint-config) are credited as referenced_dependencies; relative/absolute path entries (e.g. ./local.json) are recorded as setup_files. The logic reuses the existing is_local_specifier / is_package_specifier / resolve_config_relative_path helpers already used for jsPlugins, keeping the change minimal. A regression test covering both cases was added and passes.

Review

Verdict: SHIP

• Non-blocking: the .claude/rules/plugins.md Oxlint entry documents jsPlugins but not the new extends parsing; a one-line doc update would keep detection docs current (informational, may lag code per project convention).
• Process-only (not a code defect): commit signing and author email cannot be verified from a bare diff patch; implementer must ensure git commit -S and a valid author before push.

Closes #846

[BartWaardenburg]

Landed on main via maintainer direct-to-main flow, squashed into 27e4c3f. The linked issue is fixed on main; closing this PR as redundant.