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_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) {