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>

cmd/glint/explain.go

@@ -0,0 +1,69 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"sort"
+	"strings"
+
+	"git.k3nny.fr/glint/internal/linter"
+)
+
+func cmdExplain(args []string) {
+	if len(args) == 0 {
+		printRuleList()
+		return
+	}
+	ruleID := strings.ToUpper(args[0])
+	entry, ok := linter.RuleCatalog[ruleID]
+	if !ok {
+		fmt.Fprintf(os.Stderr, "glint explain: unknown rule %q\n\nRun 'glint explain' to list all rules.\n", ruleID)
+		os.Exit(2)
+	}
+	printRuleEntry(ruleID, entry)
+}
+
+func printRuleEntry(id string, e linter.RuleEntry) {
+	sev := strings.ToLower(string(e.Severity))
+	header := fmt.Sprintf("%s  [%s]  %s", id, sev, e.Title)
+	rule := fmt.Sprintf("\n%s\n%s\n\n%s\n",
+		header,
+		strings.Repeat("─", len(header)),
+		e.Description,
+	)
+	fmt.Print(rule)
+
+	if e.Example != "" {
+		fmt.Println("Example:")
+		fmt.Println()
+		for _, line := range strings.Split(e.Example, "\n") {
+			fmt.Printf("  %s\n", line)
+		}
+		fmt.Println()
+	}
+
+	if e.Fix != "" {
+		fmt.Println("Fix:")
+		fmt.Println()
+		for _, line := range strings.Split(e.Fix, "\n") {
+			fmt.Printf("  %s\n", line)
+		}
+		fmt.Println()
+	}
+}
+
+func printRuleList() {
+	ids := make([]string, 0, len(linter.RuleCatalog))
+	for id := range linter.RuleCatalog {
+		ids = append(ids, id)
+	}
+	sort.Strings(ids)
+
+	fmt.Printf("glint %s — lint rules\n\n", version)
+	for _, id := range ids {
+		e := linter.RuleCatalog[id]
+		sev := strings.ToLower(string(e.Severity))
+		fmt.Printf("  %-6s  [%-7s]  %s\n", id, sev, e.Title)
+	}
+	fmt.Println("\nUse 'glint explain <RULE>' for details on a specific rule.")
+}

cmd/glint/main.go

@@ -39,6 +39,7 @@ Usage: glint [OPTIONS] <COMMAND>
 Commands:
   check    Lint a pipeline file — exits 0 (clean) or 1 (errors found)
   graph    Visualise the pipeline as a job tree or Mermaid graph
+  explain  Show description and fix for a lint rule (e.g. glint explain GL007)
 
 Options:
   -h, --help     Print help
@@ -57,6 +58,8 @@ func main() {
 		cmdCheck(os.Args[2:])
 	case "graph":
 		cmdGraph(os.Args[2:])
+	case "explain":
+		cmdExplain(os.Args[2:])
 	case "-h", "--help", "help":
 		fmt.Fprintf(os.Stderr, "glint %s\n\n", version)
 		fmt.Fprint(os.Stderr, globalUsage)