chore: scrub #NNN issue references from code comments and nix descriptions

Part of the docs-migration chore (issue #708). Remove GitHub issue
numbers from inline comments, option descriptions, and rustdoc —
these are contextless noise for anyone reading the code without
access to the original discussions. Replace with prose that captures
the same rationale directly.

No functional change. Build still clean (cargo check passes).

hive-c0re/src/meta.rs

@@ -118,7 +118,7 @@ pub async fn sync_agents(
     // the gateway picks up at runtime without needing a
     // nixos-rebuild. The gateway container bind-mounts
     // /var/lib/hyperhive/ and a systemd path unit fires
-    // `nginx -s reload` when this file changes (#869). Same
+    // `nginx -s reload` when this file changes. Same
     // best-effort + non-fatal shape.
     if let Err(e) = crate::gateway_nginx::write(&agent_names) {
         tracing::warn!(error = ?e, "gateway_nginx::write failed (non-fatal)");