fix(#2164): address argus review — Option<String> secret + stale hook cleanup

Two issues flagged by argus in PR #2388 review:

1. Empty-key fallback: when load_or_generate() failed, webhook_secret was
   String::new(). An attacker knowing this could forge deliveries with a
   valid HMAC of the empty key. Fix: change to Option<String>; on None,
   skip hook registration entirely and return 503 from /webhook/* handlers
   (rather than 401 with a misleadingly-verifiable empty-key HMAC).

2. Stale hook cleanup: on upgrade from old code, old loopback hooks
   (http://127.0.0.1:.../webhook/knowledge, .../webhook/config-pr) were
   left alongside the new domain-URL hook. Fix: during ensure_webhook /
   ensure_config_pr_webhook, after listing hooks, delete any that end with
   our path suffix but point at a different base URL.

clippy + nix fmt clean.

hive-c0re/src/workers/knowledge.rs

@@ -190,6 +190,26 @@ pub async fn ensure_webhook(
                 tracing::debug!(%target_url, "knowledge: push webhook already configured");
                 return Ok(());
             }
+            // Delete stale hooks that point at our path but a different base
+            // (e.g. old loopback hooks from before the SSRF-bypass migration).
+            for h in &hooks {
+                let hook_url = h
+                    .config
+                    .as_ref()
+                    .and_then(|c| c.get("url"))
+                    .map_or("", String::as_str);
+                if hook_url.ends_with("/webhook/knowledge")
+                    && hook_url != target_url
+                    && let Some(id) = h.id
+                {
+                    tracing::info!(hook_url, "knowledge: deleting stale webhook (wrong base)");
+                    let _ = tokio::time::timeout(
+                        HTTP_TIMEOUT,
+                        client.repo_delete_hook(ORG, REPO, id).send(),
+                    )
+                    .await;
+                }
+            }
         }
         Err(e) => {
             tracing::debug!(error = %e, "knowledge: listing hooks failed; attempting create");