flake: wire hive-screen-mcp into the agent package set

`hyperhive.gui.enable = true` could not evaluate for any agent:

    error: attribute 'hive-screen-mcp' missing
    at nix/agent-modules/screen.nix:23:46

screen.nix is activated by `gui.enable` and reads
`config.hyperhive.packages.hive-screen-mcp`, but the flake's
`agentPackages` module — the thing that actually populates
`hyperhive.packages` from `self.packages.<system>` — never inherited
it. The package itself was fine (`nix/packages/default.nix` builds it,
and `nix/agent-modules/packages.nix` already documents
`hive-screen-mcp` as one of the keys it provides), so this was purely
the missing third wiring edit.

It went unnoticed because `hyperhive.packages` is a plain
`attrsOf package`: a missing key isn't a schema error, it only fails
where it's dereferenced. No agent on this hive currently enables the
GUI, so nothing ever dereferenced it.

Found when a config PR enabling the GUI for an agent failed its deploy
verify step.

flake.nix

@@ -91,6 +91,7 @@
                   hive-forge-notify
                   hive-matrix-daemon
                   hive-metric
+                  hive-screen-mcp
                   assets
                   frontend
                   reference-docs