feat(graph): code change impact graph with blast-radius queries (#65)

Implements issue #30. Builds an in-memory directed dependency graph and
answers blast-radius queries: given a set of changed files, which other
files are transitively affected?

- Graph: nodes (files/packages) + directed edges with weights
- BlastRadius: BFS over reverse edges, returns affected nodes ranked by
  impact score (1.0 at depth 1, halved per level)
- BuildFromGoFiles: walks a directory tree and extracts Go import edges
  using line-by-line parsing (no cgo/go/packages dependency)
- Stats: node/edge counts, max in/out degree, top-5 hub nodes

Co-authored-by: Ona <no-reply@ona.com>

Author: Siddhant-K-code

pkg/graph/builder.go

@@ -0,0 +1,131 @@
+package graph
+
+import (
+	"bufio"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// BuildFromGoFiles walks a directory tree and builds a dependency graph from
+// Go import statements. It does not require go/packages or cgo — it uses
+// simple line-by-line parsing of import blocks.
+func BuildFromGoFiles(root string) (*Graph, error) {
+	g := New()
+
+	err := filepath.WalkDir(root, func(path string, d os.DirEntry, err error) error {
+		if err != nil {
+			return nil // skip unreadable entries
+		}
+		if d.IsDir() {
+			// Skip hidden dirs and vendor.
+			name := d.Name()
+			if strings.HasPrefix(name, ".") || name == "vendor" || name == "testdata" {
+				return filepath.SkipDir
+			}
+			return nil
+		}
+		if !strings.HasSuffix(path, ".go") {
+			return nil
+		}
+
+		rel, err := filepath.Rel(root, path)
+		if err != nil {
+			return nil
+		}
+		rel = filepath.ToSlash(rel)
+
+		pkg := filepath.ToSlash(filepath.Dir(rel))
+		if pkg == "." {
+			pkg = ""
+		}
+
+		lang := "go"
+		tags := []string{}
+		if strings.HasSuffix(path, "_test.go") {
+			tags = append(tags, "test")
+		}
+
+		g.AddNode(Node{
+			ID:       rel,
+			Type:     NodeTypeFile,
+			Package:  pkg,
+			Language: lang,
+			Tags:     tags,
+		})
+
+		imports, err := parseGoImports(path)
+		if err != nil {
+			return nil
+		}
+
+		for _, imp := range imports {
+			// Only track intra-repo imports (those that start with the module path
+			// or are relative). We store the import path as the target node ID.
+			// Callers can resolve these to file paths if needed.
+			g.AddEdge(Edge{
+				From:   rel,
+				To:     imp,
+				Weight: 1.0,
+			})
+		}
+		return nil
+	})
+	return g, err
+}
+
+// parseGoImports extracts import paths from a Go source file.
+func parseGoImports(path string) ([]string, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	var imports []string
+	scanner := bufio.NewScanner(f)
+	inImport := false
+
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+
+		if line == "import (" {
+			inImport = true
+			continue
+		}
+		if inImport && line == ")" {
+			inImport = false
+			continue
+		}
+		if strings.HasPrefix(line, "import ") && !inImport {
+			// Single-line import.
+			imp := extractImportPath(line)
+			if imp != "" {
+				imports = append(imports, imp)
+			}
+			continue
+		}
+		if inImport && line != "" && !strings.HasPrefix(line, "//") {
+			imp := extractImportPath(line)
+			if imp != "" {
+				imports = append(imports, imp)
+			}
+		}
+	}
+	return imports, scanner.Err()
+}
+
+// extractImportPath extracts the import path string from a line like:
+//
+//	"github.com/foo/bar"
+//	alias "github.com/foo/bar"
+//	_ "github.com/foo/bar"
+func extractImportPath(line string) string {
+	// Find the quoted string.
+	start := strings.Index(line, `"`)
+	end := strings.LastIndex(line, `"`)
+	if start < 0 || end <= start {
+		return ""
+	}
+	return line[start+1 : end]
+}