feat(arcrun): implement arcrun MVP — open-source AI workflow engine

Phase 1-5 complete per .agents/specs/u6u-core-mvp/:

**Phase 1 — Cherry-pick & cleanup**
- Create arcrun/ from cypher-executor, credentials, builtins, registry
- Remove 9 InkStone Service Bindings (KBDB, REGISTRY, CLINIC_*, AICEO, MINI_ME)
- Rewrite component-loader: 3-layer (builtin → WASM_BUCKET R2 → error)
- Remove autoPublishMissing.ts, proxy.ts (AICEO), execution-logger.ts (KBDB)
- Clean all KV namespace IDs and InkStone internal URLs from config files

**Phase 2 — contract.yaml completeness**
- Add credentials_required to gmail, google_sheets, telegram, line_notify
- Add config_example to all 21 components with annotated field descriptions

**Phase 3 — Credential injection**
- Add credential-injector.ts: AES-GCM decrypt from CREDENTIALS_KV
- Integrate into GraphExecutor before WASM execution
- Structured errors with repair instructions when credential missing

**Phase 4 — CLI (acr)**
- cli/package.json: arcrun package, bin: acr, deps: commander/js-yaml/chalk/ora
- 8 commands: init, creds push, push, run, validate, parts, list, logs
- Standard mode: writes directly to user's CF KV via CF REST API
- acr init: interactive setup with arcrun.dev API Key registration

**Phase 5 — Open source release prep**
- README.md: 5-minute quickstart, component table, workflow YAML syntax
- CONTRIBUTING.md: TinyGo dev env, component scaffolding, submission flow
- Security audit: no InkStone internal URLs/IDs in committed files
- .gitignore: exclude credentials.yaml, .wrangler, *.wasm

https://claude.ai/code/session_01BnCdSLVH8tUed9VrrPavgT

registry/components/cron/main.go

@@ -0,0 +1,123 @@
+// cron — 驗證 cron expression 格式，回傳 cron_id
+// 實際排程由 Cypher Executor 負責
+//
+//go:build tinygo
+
+package main
+
+import (
+	"encoding/json"
+	"io"
+	"os"
+	"strconv"
+	"strings"
+	"time"
+)
+
+type Input struct {
+	CronExpr    string `json:"cron_expr"`
+	Description string `json:"description"`
+}
+
+func main() {
+	raw, err := io.ReadAll(os.Stdin)
+	if err != nil {
+		writeError("failed to read stdin: " + err.Error())
+		return
+	}
+	var input Input
+	if err := json.Unmarshal(raw, &input); err != nil {
+		writeError("invalid input JSON: " + err.Error())
+		return
+	}
+	if input.CronExpr == "" {
+		writeError("cron_expr 必填")
+		return
+	}
+
+	if err := validateCronExpr(input.CronExpr); err != nil {
+		writeError("無效的 cron expression: " + err.Error())
+		return
+	}
+
+	cronID := "cron-" + strconv.FormatInt(time.Now().UnixNano()/1e6, 10)
+
+	out, _ := json.Marshal(map[string]interface{}{
+		"success": true,
+		"data": map[string]interface{}{
+			"cron_id":     cronID,
+			"cron_expr":   input.CronExpr,
+			"enabled":     true,
+			"description": input.Description,
+		},
+	})
+	os.Stdout.Write(out)
+}
+
+// validateCronExpr — 驗證標準 5 欄位 cron expression
+func validateCronExpr(expr string) error {
+	fields := strings.Fields(expr)
+	if len(fields) != 5 {
+		return &cronError{"需要 5 個欄位（分 時 日 月 週），實際: " + strconv.Itoa(len(fields))}
+	}
+
+	// 各欄位範圍: 分(0-59), 時(0-23), 日(1-31), 月(1-12), 週(0-7)
+	ranges := [][2]int{{0, 59}, {0, 23}, {1, 31}, {1, 12}, {0, 7}}
+	names := []string{"分鐘", "小時", "日", "月", "星期"}
+
+	for i, field := range fields {
+		if err := validateCronField(field, ranges[i][0], ranges[i][1], names[i]); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func validateCronField(field string, min, max int, name string) error {
+	if field == "*" {
+		return nil
+	}
+	// 支援 */n 格式
+	if strings.HasPrefix(field, "*/") {
+		n, err := strconv.Atoi(field[2:])
+		if err != nil || n <= 0 {
+			return &cronError{name + " 步進值無效: " + field}
+		}
+		return nil
+	}
+	// 支援 a-b 範圍
+	if strings.Contains(field, "-") {
+		parts := strings.SplitN(field, "-", 2)
+		a, err1 := strconv.Atoi(parts[0])
+		b, err2 := strconv.Atoi(parts[1])
+		if err1 != nil || err2 != nil || a < min || b > max || a > b {
+			return &cronError{name + " 範圍無效: " + field}
+		}
+		return nil
+	}
+	// 支援逗號分隔
+	if strings.Contains(field, ",") {
+		for _, part := range strings.Split(field, ",") {
+			n, err := strconv.Atoi(part)
+			if err != nil || n < min || n > max {
+				return &cronError{name + " 值無效: " + part}
+			}
+		}
+		return nil
+	}
+	// 單一數字
+	n, err := strconv.Atoi(field)
+	if err != nil || n < min || n > max {
+		return &cronError{name + " 值超出範圍 [" + strconv.Itoa(min) + "-" + strconv.Itoa(max) + "]: " + field}
+	}
+	return nil
+}
+
+type cronError struct{ msg string }
+
+func (e *cronError) Error() string { return e.msg }
+
+func writeError(msg string) {
+	out, _ := json.Marshal(map[string]interface{}{"success": false, "error": msg})
+	os.Stdout.Write(out)
+}