feat: add CSS @import tracking and path alias resolution to dependency graph

[cstuncsik]

Summary

Follow-up to #9 (Svelte/Vue import parsing), addressing both known limitations listed there:

1. Path aliases ($lib/, @/, ~/) were treated as external packages — now resolved via tsconfig.json / jsconfig.json compilerOptions.paths
2. CSS @import in <style> blocks was not tracked — now extracted from Svelte and Vue <style> blocks (any combination of lang, scoped, module, global attributes), plus standalone .css/.scss/.sass/.less files

Both features add zero new dependencies, reusing existing ast-grep HTML parsing (same approach as #9) and the resolveRelativePath infrastructure.

How it works

CSS @import extraction:

1. In Svelte/Vue files: after extracting <script> imports, also finds <style> blocks via style_element AST nodes and applies a regex to extract @import/@import url(...) statements (CSS/SCSS/LESS) and @require (Stylus)
2. For standalone CSS/SCSS/SASS/LESS files: maps them to Lang.Css (lenient parser) and applies the same regex extractor
3. CSS imports are tagged with isCssImport: true so the graph builder routes them to CSS resolution (with .css/.scss/.sass/.less/.styl extensions) instead of JS resolution — critical for Svelte/Vue files where script and style imports coexist
4. SCSS partial resolution: @import "variables" correctly finds _variables.scss (underscore-prefix convention)

Path alias resolution:

1. loadPathAliases() reads tsconfig.json (falling back to jsconfig.json) and parses compilerOptions.paths + baseUrl
2. Follows extends chains (up to 10 levels, with circular reference protection) to find paths in parent configs — common in monorepo setups
3. Converts wildcard patterns ("$lib/*": ["src/lib/*"]) to prefix→directory mappings
4. Exact patterns ("~": ["./src"]) match only the exact specifier (not prefix-based)
5. Aliases are loaded once per graph build and passed through to resolveImport()
6. Works for both JS/TS/Svelte/Vue imports and CSS imports (e.g., $lib/styles/vars.css)

Changes

src/services/graph-aliases.ts (new):

• loadPathAliases() — reads tsconfig/jsconfig, follows extends chains
• parsePathAliases() — parses compilerOptions.paths with baseUrl support
• parseTsconfigJson() — JSON parser with comment stripping that preserves string values
• followExtendsChain() — walks extends references with circular reference and depth protection

src/services/graph-imports.ts:

• Added extractCssImports() — regex-based extraction of @import, @import url(...), and Stylus @require
• Added isCssImport flag to ImportInfo interface
• Svelte/Vue handler: extracts CSS imports from <style> blocks after script extraction
• Added "Css" case in AST switch for standalone CSS/SCSS/SASS/LESS files

src/services/graph-resolution.ts:

• Added aliases parameter to resolveImport()
• Added resolveAliasPath() helper with wildcard vs exact prefix matching
• Added css/scss/sass/less language cases with CSS extension resolution
• Added SCSS partial resolution (_ prefix) in resolveRelativePath()

src/services/code-graph.ts:

• Loads path aliases once per graph build via loadPathAliases()
• Routes isCssImport imports to CSS resolution language
• Added .sass and .less to getAstGrepLang() mapping

Known limitations

• Stylus bare-identifier @require (without quotes) is not supported — quoted form only
• tsconfig.extends with package references (e.g., @tsconfig/node20) resolves via local node_modules/ — may not work with hoisted packages in some monorepo setups
• CSS @import with bare identifiers (no quotes) is not supported — very rare in practice

Test plan

• All 714 existing + new tests pass
• TypeScript build passes with no errors
• 21 tests for alias loading/parsing (tests/unit/graph-aliases.test.ts): tsconfig paths, baseUrl, jsconfig fallback, extends chains (single/multi-level), circular extends, missing files, comment stripping, malformed targets
• 48 tests for import extraction (tests/unit/graph-imports.test.ts): CSS @import in Svelte/Vue style blocks (all attribute combinations), Stylus @require, standalone CSS, external URL filtering, isCssImport flag
• 63 tests for resolution (tests/unit/graph-resolution.test.ts): alias resolution (wildcard/exact), SCSS partials (_prefix), CSS/SCSS/SASS/LESS resolution, alias + partial combined, backwards compatibility without aliases

[cstuncsik]

@pineapplestrikesback, you might also want to have a look at this