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).

nix/modules/hive-c0re.nix

@@ -75,7 +75,7 @@ in
       canonical readable identity. Exposed to agents as
       `HYPERHIVE_HIVE_NAME`; surfaced in the dashboard chrome and
       per-agent system prompt when set. Null falls back to the
-      pre-#701 behaviour (chrome shows the domain, prompt doesn't
+      default behaviour (chrome shows the domain, prompt doesn't
       mention a hive name).
     '';
   };
@@ -141,7 +141,7 @@ in
       description = ''
         hyperhive workspace package. Provides `/bin/hive-c0re`
         (coordinator daemon + admin-socket CLI) and `/bin/hivectl`
-        (operator-facing host CLI for ad-hoc administration; #655).
+        (operator-facing host CLI for ad-hoc administration).
       '';
     };
     frontend = lib.mkOption {