Revert "Switch CI from Vira self-hosted to GitHub Actions (#40)"

Remove the GitHub Actions CI workflow, the `just ci` target, and
restore explicit build.systems in vira.hs so Vira self-hosted handles
multi-platform builds again.

This also effectively reverts #41 (CI trigger on flake update bot
branch) since the workflow file is removed entirely.

Author: srid

.github/workflows/ci.yml

@@ -2,10 +2,6 @@
 default:
     @just --list
 
-# Run CI locally
-ci:
-    vira ci -b
-
 # Record the demo screencast (requires JUSPAY_API_KEY)
 demo:
     nix run ./demo --override-input oc . -- coding-agents/opencode/demo.tape

justfile

@@ -5,7 +5,11 @@
     isMain = ctx.branch == "main"
   in
   pipeline
-    { build.flakes =
+    { build.systems =
+        [ "x86_64-linux"
+        , "aarch64-darwin"
+        ]
+    , build.flakes =
         [ "."
         , "./demo" { overrideInputs = [("oc", ".")] }
         , "./coding-agents/opencode/test/home-manager" { overrideInputs = [("oc", ".")] }