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/agent_sockets.rs

@@ -38,7 +38,7 @@ pub const SOCKET_FILENAME: &str = "web.sock";
 /// sub-agent that hasn't flipped the option yet.
 ///
 /// Renamed from `.bound` (legacy) to match the `hyperhive-` prefix
-/// convention for all harness-written state files (#838). `build_map`
+/// convention for all harness-written state files. `build_map`
 /// checks both names during the transition window so existing containers
 /// don't lose gateway routing before their next rebuild.
 pub const READY_MARKER: &str = "hyperhive-socket-bound";