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/tools.go

@@ -0,0 +1,29 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/urfave/cli/v2"
+
+	cmds "gitea-mail-templates/tools/cli"
+)
+
+func main() {
+	app := &cli.App{
+		Name:      "tools",
+		Usage:     "Gitea Mail Templates CLI — manage and preview email template themes",
+		UsageText: "tools <command> [args...] [--flags]",
+		Commands: []*cli.Command{
+			cmds.ListCommand(),
+			cmds.CreateCommand(),
+			cmds.DeleteCommand(),
+			cmds.PreviewCommand(),
+		},
+	}
+
+	if err := app.Run(os.Args); err != nil {
+		fmt.Fprintf(os.Stderr, "error: %v\n", err)
+		os.Exit(1)
+	}
+}