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/dashboard_events.rs

@@ -178,7 +178,7 @@ pub enum DashboardEvent {
     /// immediately and runs the `nix flake update` + agent-rebuild
     /// ripple in a background task — this event lets the META INPUTS
     /// panel show a disabled "updating…" state for that whole window
-    /// instead of looking idle (issue #259). Emitted by
+    /// instead of looking idle. Emitted by
     /// `Coordinator::meta_update_guard` / `MetaUpdateGuard::drop` only
     /// when the active-run count crosses 0, so concurrent updates flip
     /// the flag exactly once.
@@ -198,8 +198,8 @@ impl DashboardEvent {
     /// Snake-case identifier matching this variant's serde `tag`
     /// (e.g. `Sent` → `"sent"`, `ContainerStateChanged` →
     /// `"container_state_changed"`). Lets `/dashboard/stream`'s
-    /// `?kinds=` filter (#408) decide whether to forward a frame
-    /// without paying the JSON-serialise cost first.
+    /// `?kinds=` filter decide whether to forward a frame without
+    /// paying the JSON-serialise cost first.
     ///
     /// Keep in sync with `#[serde(rename_all = "snake_case", tag =
     /// "kind")]` on `DashboardEvent` — if a new variant lands above,