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/model/pipeline_test.go

@@ -0,0 +1,22 @@
+package model
+
+import (
+	"testing"
+)
+
+func TestSetJobOrigin(t *testing.T) {
+	p := &Pipeline{
+		Jobs: map[string]Job{
+			"with-file": {Name: "with-file", File: "existing.yml"},
+			"no-file":   {Name: "no-file"},
+		},
+	}
+	p.SetJobOrigin("default.yml")
+
+	if p.Jobs["with-file"].File != "existing.yml" {
+		t.Error("job with existing File should not be overwritten")
+	}
+	if p.Jobs["no-file"].File != "default.yml" {
+		t.Errorf("job without File should be set: got %q", p.Jobs["no-file"].File)
+	}
+}