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/linter/if_reachability_test.go

@@ -150,3 +150,12 @@ func TestCheckRulesIfReachability(t *testing.T) {
 		})
 	}
 }
+
+// TestCheckRulesIfReachability_EmptyPipeline covers the early return (line 16-18)
+// when the pipeline has no jobs.
+func TestCheckRulesIfReachability_EmptyPipeline(t *testing.T) {
+	findings := checkRulesIfReachability(&model.Pipeline{Jobs: map[string]model.Job{}})
+	if len(findings) != 0 {
+		t.Errorf("empty pipeline: expected no findings, got %v", findings)
+	}
+}