fix: resolve package subpath exports in import graph

[pkudinov]

Summary

This PR fixes Graphify import resolution for package subpath exports that point at source files in monorepos using package.json exports with subpath keys.

What this changes

• In graphify/extract.py, resolves subpath export target strings through _package_export_target_values.
• Added array-form support for export targets so both "./foo.ts" and ["./foo.ts", "./foo.dev.ts"] forms are handled recursively.
• Extended tests/test_js_import_resolution.py with a regression test covering pnpm workspace imports of package subpath exports.

Why this matters for tokenbooks

packages/accounting uses package subpath exports (for browser/server split) and Graphify currently recorded the target as browser instead of resolving through package.json to a source file path.
That prevented accurate Jest test impact and could over-run package-level test selection.

Verification

• uv run pytest -q
  • Result: 2098 passed, 28 skipped.

Notes

This also resolves the rebase conflict from prior comment context by preserving both npm/yarn and pnpm import scenarios in existing test coverage.

[pkudinov]

Closing this as a duplicate. I pushed the same rebased fix to the original PR: #1333.