feat: add event type and action filtering

Configure which events to forward via comma-separated 'events' config.
Supports event-level ('push', 'pull_request') or event:action-level
('pull_request:opened', 'issues:closed') filtering.

Empty/omitted = forward everything (backwards compatible).

filter_test.go

@@ -0,0 +1,32 @@
+package main
+
+import "testing"
+
+func TestEventFilter(t *testing.T) {
+	tests := []struct {
+		name    string
+		filters []string
+		event   string
+		action  string
+		want    bool
+	}{
+		{"empty allows all", nil, "push", "", true},
+		{"event only", []string{"push"}, "push", "", true},
+		{"event only blocks other", []string{"push"}, "issues", "opened", false},
+		{"event:action match", []string{"pull_request:opened"}, "pull_request", "opened", true},
+		{"event:action no match", []string{"pull_request:opened"}, "pull_request", "closed", false},
+		{"multiple actions", []string{"pull_request:opened", "pull_request:closed"}, "pull_request", "closed", true},
+		{"mixed event and action", []string{"push", "pull_request:opened"}, "push", "", true},
+		{"mixed blocks filtered", []string{"push", "pull_request:opened"}, "pull_request", "closed", false},
+		{"case insensitive", []string{"Pull_Request:Opened"}, "pull_request", "opened", true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			ef := ParseEventFilter(tt.filters)
+			if got := ef.Allowed(tt.event, tt.action); got != tt.want {
+				t.Errorf("Allowed(%q, %q) = %v, want %v", tt.event, tt.action, got, tt.want)
+			}
+		})
+	}
+}