feat(cli): output formats, GL034-GL041 lint rules, include inputs and cache

Bundles three patch releases (v0.2.16–v0.2.18):

v0.2.18 — output formats (--format flag on glint check):
- json: stable JSON report (schema_version: 1, findings array, summary)
- sarif: SARIF 2.1.0 for GitHub Code Scanning / GitLab SAST
- junit: JUnit XML for CI test-report artifacts (artifacts:reports:junit)
- github: GitHub Actions ::error:: / ::warning:: annotation lines
- Unknown --format value exits 2 with a helpful error message
- Summary line routed to stderr in structured formats; context suppressed

v0.2.17 — include resolution improvements:
- Recursive include depth capped at 100 (matches GitLab's own limit)
- project: and component: includes tracked in visited set (cycle detection)
- $[[ inputs.KEY ]] / $[[ inputs.KEY | default(…) ]] substituted from with:
- --cache-dir: persist fetched remote templates to disk (SHA-256 keyed)
- --offline: serve from cache only; defaults to ~/.cache/glint

v0.2.16 — new lint rules (GL034–GL041):
- GL034: services map form requires name; alias must be valid DNS label
- GL035: rules:changes / rules:exists absolute path detection
- GL036: timeout format validation (job-level + default.timeout)
- GL037: id_tokens entries must have an aud key
- GL038: secrets entries must declare a provider (vault / gcp / azure)
- GL039: pages: keyword + artifacts.paths consistency
- GL040: duplicate stage names in stages: list
- GL041: cache.key.files must be exact paths, not globs

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

internal/linter/linter.go

@@ -55,6 +55,8 @@ func (f Finding) String() string {
 func Lint(p *model.Pipeline) []Finding {
 	var findings []Finding
 	findings = append(findings, checkStages(p)...)
+	findings = append(findings, checkDuplicateStages(p)...)
+	findings = append(findings, checkDefault(p)...)
 	findings = append(findings, checkWorkflow(p)...)
 	findings = append(findings, checkJobs(p)...)
 	findings = append(findings, checkNeeds(p)...)
@@ -85,6 +87,32 @@ func checkStages(p *model.Pipeline) []Finding {
 	return findings
 }
 
+// GL040: warn when a stage name appears more than once in stages:.
+func checkDuplicateStages(p *model.Pipeline) []Finding {
+	seen := make(map[string]bool, len(p.Stages))
+	var findings []Finding
+	for _, s := range p.Stages {
+		if seen[s] {
+			findings = append(findings, Finding{
+				Severity: Warning,
+				Rule:     RuleDuplicateStage,
+				File:     p.SourceFile,
+				Message:  fmt.Sprintf("stage %q appears more than once in 'stages'; GitLab silently merges duplicate stage entries", s),
+			})
+		}
+		seen[s] = true
+	}
+	return findings
+}
+
+// checkDefault validates the pipeline-level default: block.
+func checkDefault(p *model.Pipeline) []Finding {
+	if p.Default == nil {
+		return nil
+	}
+	return checkDefaultTimeout(p.Default.Timeout, p.SourceFile)
+}
+
 func checkWorkflow(p *model.Pipeline) []Finding {
 	if p.Workflow == nil {
 		return nil