test(coverage): add unit tests across all packages; remove dead code

- Added comprehensive table-driven test suites for all packages:
  cmd/glint, cicontext, fetcher, graph, linter, model, resolver.
  Coverage reaches 98%+ statement coverage across the codebase.
- Replaced os.Exit calls in cmd/glint with an `exit` variable so tests
  can capture exit codes without terminating the test process.
- Removed unreachable code found during coverage analysis:
  dead guard in cicontext.parseRegexLiteral; dead len(jobs)==0 branch
  in graph.Pipeline; skipWin struct field and dead continue in
  graph.convertToPNG; pipelineSVG return type simplified to string.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/fetcher/cache_test.go

@@ -0,0 +1,61 @@
+package fetcher
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestCachePath(t *testing.T) {
+	p1 := cachePath("/tmp/cache", "key1")
+	p2 := cachePath("/tmp/cache", "key2")
+	if p1 == p2 {
+		t.Error("different keys should produce different paths")
+	}
+	if filepath.Dir(p1) != "/tmp/cache" {
+		t.Errorf("expected /tmp/cache dir, got %q", filepath.Dir(p1))
+	}
+}
+
+func TestCacheReadMiss(t *testing.T) {
+	// empty dir → miss
+	data, ok := cacheRead("", "key")
+	if ok || data != nil {
+		t.Error("empty cacheDir should always miss")
+	}
+
+	// nonexistent entry → miss
+	data, ok = cacheRead(t.TempDir(), "nonexistent-key")
+	if ok || data != nil {
+		t.Error("missing cache entry should miss")
+	}
+}
+
+func TestCacheWriteAndRead(t *testing.T) {
+	dir := t.TempDir()
+	key := "https://example.com/template.yml"
+	content := []byte("stages: [build]")
+
+	cacheWrite(dir, key, content)
+
+	got, ok := cacheRead(dir, key)
+	if !ok {
+		t.Fatal("expected cache hit after write")
+	}
+	if string(got) != string(content) {
+		t.Errorf("cached content mismatch: got %q want %q", got, content)
+	}
+}
+
+func TestCacheWrite_EmptyDir(t *testing.T) {
+	// Should silently do nothing when dir is empty string.
+	cacheWrite("", "key", []byte("data"))
+}
+
+func TestCacheWrite_MkdirAll(t *testing.T) {
+	dir := filepath.Join(t.TempDir(), "sub", "dir")
+	cacheWrite(dir, "k", []byte("v"))
+	if _, err := os.Stat(dir); err != nil {
+		t.Errorf("directory not created: %v", err)
+	}
+}