fix: resolve review issues for Slack target type

- Fix duplicate name="url" form fields by adding :disabled binding
  to each URL input so only the active target type's field submits
- Fix message formatting to use Slack mrkdwn (*bold*) instead of
  Markdown (**bold**) and remove language hint from code blocks
- Fix README line exceeding 80-column hard wrap

internal/delivery/engine.go

@@ -1080,11 +1080,11 @@ func (e *Engine) parseSlackConfig(configJSON string) (*SlackTargetConfig, error)
 func FormatSlackMessage(event *database.Event) string {
 	var b strings.Builder
 
-	b.WriteString("**Webhook Event Received**\n")
-	b.WriteString(fmt.Sprintf("**Method:** `%s`\n", event.Method))
-	b.WriteString(fmt.Sprintf("**Content-Type:** `%s`\n", event.ContentType))
-	b.WriteString(fmt.Sprintf("**Timestamp:** `%s`\n", event.CreatedAt.UTC().Format(time.RFC3339)))
-	b.WriteString(fmt.Sprintf("**Body Size:** %d bytes\n", len(event.Body)))
+	b.WriteString("*Webhook Event Received*\n")
+	b.WriteString(fmt.Sprintf("*Method:* `%s`\n", event.Method))
+	b.WriteString(fmt.Sprintf("*Content-Type:* `%s`\n", event.ContentType))
+	b.WriteString(fmt.Sprintf("*Timestamp:* `%s`\n", event.CreatedAt.UTC().Format(time.RFC3339)))
+	b.WriteString(fmt.Sprintf("*Body Size:* %d bytes\n", len(event.Body)))
 
 	if event.Body == "" {
 		b.WriteString("\n_(empty body)_\n")
@@ -1096,7 +1096,7 @@ func FormatSlackMessage(event *database.Event) string {
 	if json.Unmarshal([]byte(event.Body), &parsed) == nil {
 		var pretty bytes.Buffer
 		if json.Indent(&pretty, parsed, "", "  ") == nil {
-			b.WriteString("\n```json\n")
+			b.WriteString("\n```\n")
 			prettyStr := pretty.String()
 			// Truncate very large payloads to keep Slack messages reasonable
 			const maxPayloadDisplay = 3500

internal/delivery/engine_test.go

@@ -973,10 +973,11 @@ func TestFormatSlackMessage_JSONBody(t *testing.T) {
 
 	msg := FormatSlackMessage(event)
 
-	assert.Contains(t, msg, "**Webhook Event Received**")
+	assert.Contains(t, msg, "*Webhook Event Received*")
 	assert.Contains(t, msg, "`POST`")
 	assert.Contains(t, msg, "`application/json`")
-	assert.Contains(t, msg, "```json")
+	assert.Contains(t, msg, "```")
+	assert.NotContains(t, msg, "```json")
 	// Pretty-printed JSON should have indentation
 	assert.Contains(t, msg, `  "action": "push"`)
 	assert.Contains(t, msg, `  "repo": "test/repo"`)
@@ -994,7 +995,7 @@ func TestFormatSlackMessage_NonJSONBody(t *testing.T) {
 
 	msg := FormatSlackMessage(event)
 
-	assert.Contains(t, msg, "**Webhook Event Received**")
+	assert.Contains(t, msg, "*Webhook Event Received*")
 	assert.Contains(t, msg, "```\nhello world plain text\n```")
 	// Should NOT have ```json marker for non-JSON
 	assert.NotContains(t, msg, "```json")
@@ -1094,8 +1095,10 @@ func TestDeliverSlack_Success(t *testing.T) {
 	// Verify the Slack payload contains the expected message
 	var slackPayload map[string]string
 	require.NoError(t, json.Unmarshal([]byte(receivedBody), &slackPayload))
-	assert.Contains(t, slackPayload["text"], "**Webhook Event Received**")
-	assert.Contains(t, slackPayload["text"], "```json")
+	assert.Contains(t, slackPayload["text"], "*Webhook Event Received*")
+	assert.NotContains(t, slackPayload["text"], "**Webhook Event Received**")
+	assert.Contains(t, slackPayload["text"], "```")
+	assert.NotContains(t, slackPayload["text"], "```json")
 }
 
 func TestDeliverSlack_Failure(t *testing.T) {