feat(acp/handlers): initialize + session/new param builders

internal/acp/handlers/client_session_new.go

@@ -0,0 +1,17 @@
+package handlers
+
+// session/new method types (spec §5.4).
+
+type SessionNewParams struct {
+	Cwd        string   `json:"cwd"`
+	McpServers []string `json:"mcpServers"` // MVP: no MCP, send empty slice
+}
+
+type SessionNewResult struct {
+	SessionID string `json:"sessionId"`
+}
+
+// BuildSessionNewParams returns standard session/new params.
+func BuildSessionNewParams(cwd string) SessionNewParams {
+	return SessionNewParams{Cwd: cwd, McpServers: []string{}}
+}