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/needs_test.go

@@ -0,0 +1,38 @@
+package linter
+
+import (
+	"testing"
+
+	"git.k3nny.fr/glint/internal/model"
+)
+
+// TestCheckNeeds_StageOrder verifies that a job needing a job in a later stage
+// produces RuleNeedsStageOrder (line 64-76 in needs.go).
+func TestCheckNeeds_StageOrder(t *testing.T) {
+	p := &model.Pipeline{
+		Stages: []string{"build", "test", "deploy"},
+		Jobs: map[string]model.Job{
+			"build-job": {
+				Name:   "build-job",
+				Stage:  "build",
+				Script: []any{"make"},
+				Needs:  []any{"deploy-job"},
+			},
+			"deploy-job": {
+				Name:   "deploy-job",
+				Stage:  "deploy",
+				Script: []any{"make deploy"},
+			},
+		},
+	}
+	findings := checkNeeds(p)
+	var gotStageOrder bool
+	for _, f := range findings {
+		if f.Rule == RuleNeedsStageOrder {
+			gotStageOrder = true
+		}
+	}
+	if !gotStageOrder {
+		t.Errorf("build-job needing deploy-job (later stage): expected GL025 finding; got: %v", findings)
+	}
+}