Send the build version in the outbound User-Agent (closes #313)

The http and slack targets sent the fixed User-Agent webhooker/1.0. The
delivery engine now takes the version from Globals, the value the web UI
footer shows, and one method builds webhooker/<version> for both
targets. The http target still sets it after the configured headers, so
a configured or inbound User-Agent never reaches the wire.

A test builds the engine through its real constructor with a known
version and checks the header each target sends.

Model: opus-5-5

internal/delivery/target_slack.go

@@ -136,7 +136,7 @@ func (t *slackTarget) attempt(
 	}
 
 	req.Header.Set("Content-Type", "application/json")
-	req.Header.Set("User-Agent", "webhooker/1.0")
+	req.Header.Set("User-Agent", t.eng.userAgent())
 
 	resp, doErr := executeHTTPRequest(t.client, req)
 	durationMs := time.Since(start).Milliseconds()