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

templates/source_detail.html

@@ -98,14 +98,14 @@
                         </select>
                     </div>
                     <div x-show="targetType === 'http'">
-                        <input type="url" name="url" placeholder="https://example.com/webhook" class="input text-sm">
+                        <input type="url" name="url" placeholder="https://example.com/webhook" :disabled="targetType !== 'http'" class="input text-sm">
                     </div>
                     <div x-show="targetType === 'http'" class="flex gap-2 items-center">
                         <label class="text-sm text-gray-700">Max retries (0 = fire-and-forget):</label>
                         <input type="number" name="max_retries" value="0" min="0" max="20" class="input text-sm w-24">
                     </div>
                     <div x-show="targetType === 'slack'">
-                        <input type="url" name="url" placeholder="https://hooks.slack.com/services/..." class="input text-sm">
+                        <input type="url" name="url" placeholder="https://hooks.slack.com/services/..." :disabled="targetType !== 'slack'" class="input text-sm">
                         <p class="text-xs text-gray-500 mt-1">Slack or Mattermost incoming webhook URL. Payloads are pretty-printed in code blocks.</p>
                     </div>
                     <button type="submit" class="btn-primary text-sm">Add Target</button>