feat(linter): glint explain, GL042 rules:if: reachability, GL043 inherit completeness

- `glint explain <RULE>`: new subcommand printing rule description,
  rationale, bad-YAML example and fix for every GL001–GL043 rule.
  `glint explain` (no arg) lists all rules with ID, severity, title.
  Rule IDs are case-insensitive.

- GL042 (rules:if: evaluated reachability): warns when every rules:if:
  condition evaluates to false given the values of variables declared in
  the pipeline YAML, making the job statically unreachable. Conservative:
  only fires when all referenced variables are declared in YAML; predefined
  CI_* / GITLAB_* variables are skipped to avoid false positives.

- GL043 (inherit: completeness): warns when inherit: default: is declared
  but there is no default: block in the pipeline (dead declaration), or
  when the list form names fields not set in the default: block.

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

internal/linter/if_reachability.go

@@ -0,0 +1,104 @@
+package linter
+
+import (
+	"git.k3nny.fr/glint/internal/cicontext"
+	"git.k3nny.fr/glint/internal/model"
+)
+
+// checkRulesIfReachability (GL042): warn when every rule in a job's rules: block
+// can be statically proven to never activate given the values of variables
+// declared in the pipeline YAML. Only fires when ALL variables referenced by the
+// rules:if: expressions are declared in the pipeline or job variables: blocks —
+// predefined CI_* / GITLAB_* variables or variables not declared in YAML are
+// treated as unknown (could have any runtime value) so the check is skipped
+// conservatively to avoid false positives.
+func checkRulesIfReachability(p *model.Pipeline) []Finding {
+	if len(p.Jobs) == 0 {
+		return nil
+	}
+
+	// Collect scalar string values from pipeline-level variables.
+	pipelineVars := make(map[string]string, len(p.Variables))
+	for k, v := range p.Variables {
+		if s, ok := cicontext.ScalarString(v); ok {
+			pipelineVars[k] = s
+		}
+	}
+
+	var findings []Finding
+	for name, job := range p.Jobs {
+		if len(job.Rules) == 0 {
+			continue
+		}
+
+		// Merge pipeline vars with job-level variable overrides.
+		jobVars := make(map[string]string, len(pipelineVars)+len(job.Variables))
+		for k, v := range pipelineVars {
+			jobVars[k] = v
+		}
+		for k, v := range job.Variables {
+			if s, ok := cicontext.ScalarString(v); ok {
+				jobVars[k] = s
+			}
+		}
+
+		if f := evalRulesReachability(name, job, jobVars); f != nil {
+			findings = append(findings, *f)
+		}
+	}
+	return findings
+}
+
+// evalRulesReachability returns a GL042 finding when the job's rules: block
+// can never activate given the provided variable map, or nil if the job
+// might activate (or the check cannot be applied conservatively).
+func evalRulesReachability(name string, job model.Job, jobVars map[string]string) *Finding {
+	lookup := func(k string) string { return jobVars[k] }
+
+	for _, rule := range job.Rules {
+		// Explicit when: never — this rule is provably dead; continue.
+		if rule.When == "never" {
+			continue
+		}
+
+		// No if: condition → this rule always matches → job CAN activate.
+		if rule.If == "" {
+			return nil
+		}
+
+		// Check whether all variables referenced in the if: expression are
+		// declared in the pipeline YAML. If any are not declared (predefined
+		// CI vars, project settings, etc.), we cannot evaluate the expression
+		// and must conservatively assume the job might activate.
+		refs := extractIfVars(rule.If)
+		allDeclared := true
+		for _, ref := range refs {
+			if _, ok := jobVars[ref]; !ok {
+				// Variable not in YAML (predefined or unknown) → cannot evaluate.
+				allDeclared = false
+				break
+			}
+		}
+		if !allDeclared {
+			return nil // conservative: job might activate
+		}
+
+		// All referenced variables are declared; evaluate the expression.
+		// Use strict mode (unparse → false) to avoid matching unparseable expressions.
+		if cicontext.EvalIfStrict(rule.If, lookup) {
+			// This rule's condition evaluates to true → job CAN activate.
+			return nil
+		}
+		// Expression evaluated to false → this rule cannot activate; continue.
+	}
+
+	// No rule could activate the job.
+	return &Finding{
+		Severity: Warning,
+		Rule:     RuleStaticDeadRules,
+		Job:      name,
+		File:     job.File,
+		Line:     job.Line,
+		Message:  "rules: block can never activate: all if: conditions evaluate to false given the declared pipeline variables",
+	}
+}