refactor(tools): modularize CLI with urfave/cli/v2, externalise template metadata

- Split monolithic build-preview.go into modular packages:
  tools.go (entry), cli/ (list/create/delete/preview commands),
  config/ (JSON loading), preview/ (rendering engine, funcs, locale)
- Extract all template metadata into data/templates_config.json
  as single source of truth — no hardcoded template data in Go
- Replace hand-rolled CLI parsing with github.com/urfave/cli/v2
- Add sensible defaults for --folder (../themes) and --config
  (./data/templates_config.json) — most commands now run bare
- Add type-coercing gt/lt/ge/le template funcs for JSON float64
- Fix AppUrl to gitea.com matching official template convention
- rendered.js now exports __REGISTRY__ and __PARAMS__ alongside
  __RENDERED__ — preview/index.html reads all from single source
- Cross-validate all 11 template data contexts against official
  Gitea source (services/mailer/*.go + templates/mail/*.tmpl)
- Update all docs (README, AGENTS, CONTRIBUTING, 5 languages)
  to reflect new CLI workflow with create scaffolding command

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/cli/delete.go

@@ -0,0 +1,51 @@
+package cli
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/urfave/cli/v2"
+)
+
+// DeleteCommand returns the "delete" subcommand.
+func DeleteCommand() *cli.Command {
+	return &cli.Command{
+		Name:      "delete",
+		Usage:     "Delete one or more theme style directories",
+		UsageText: "go run . delete [--folder <themes-dir>] <style-name>...",
+		Flags: []cli.Flag{
+			&cli.StringFlag{
+				Name:  "folder",
+				Value: "../themes",
+				Usage: "Path to the themes directory",
+			},
+		},
+		Action: runDelete,
+	}
+}
+
+func runDelete(c *cli.Context) error {
+	names := c.Args().Slice()
+	if len(names) == 0 {
+		return fmt.Errorf("at least one style name is required")
+	}
+
+	folder := c.String("folder")
+
+	for _, styleName := range names {
+		styleDir := filepath.Join(folder, styleName)
+
+		if _, err := os.Stat(styleDir); os.IsNotExist(err) {
+			fmt.Printf("  [skip] '%s' does not exist\n", styleName)
+			continue
+		}
+
+		if err := os.RemoveAll(styleDir); err != nil {
+			return fmt.Errorf("cannot delete %s: %w", styleDir, err)
+		}
+		fmt.Printf("  Deleted style '%s'\n", styleName)
+	}
+
+	return nil
+}