fix(nix): wire hive-forge-notify into the agent package set

The new daemon's package was added to `nix/packages/default.nix` and
referenced from the forge agent module, but not to the `inherit` list in
the flake's `agentPackages` module — so `hyperhive.packages` never gained
the attribute and every agent container failed to evaluate.

Also drop the crate's `[[bin]]` section: the binary name defaults to the
package name and the path to `src/main.rs`, so all of it was restating
cargo's defaults. The comment explaining why this is a separate process
moves to the top of the manifest, where it isn't attached to a section
that no longer exists.

flake.nix

@@ -88,6 +88,7 @@
                   hive-agent-mcp
                   hive-bash-daemon
                   hive-forge
+                  hive-forge-notify
                   hive-matrix-daemon
                   hive-metric
                   assets