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/graph/pipeline.go

@@ -82,9 +82,6 @@ func Pipeline(p *model.Pipeline) string {
 	// Emit one subgraph per stage.
 	for _, stage := range stages {
 		jobs := byStage[stage]
-		if len(jobs) == 0 {
-			continue
-		}
 		sort.Strings(jobs)
 		wf("    subgraph %s[\"%s\"]", stageID(stage), stage)
 		for _, name := range jobs {