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

README.md

@@ -611,8 +611,8 @@ fine — startup recovery rescans the database anyway).
 
 **Scope:** Circuit breakers only apply to **HTTP targets with
 `max_retries` > 0**. Fire-and-forget HTTP targets (`max_retries` == 0),
-Slack targets, database targets (local operations), and log targets (stdout) do not use
-circuit breakers.
+Slack targets, database targets (local operations), and log
+targets (stdout) do not use circuit breakers.
 
 When a circuit is open and a new delivery arrives, the engine marks the
 delivery as `retrying` and schedules a retry timer for after the